Class 10 Science · Chapter 8

Heredity

285 Class 10 Science questions on Heredity from the NCERT textbook, NCERT Exemplar and CBSE board papers 2017–2026, each with its answer and a step-by-step solution.

NCERT exercises

Exercises (4) Intext Questions (Page 129) (2) Intext Questions (Page 133) (4)

NCERT Exemplar exercises

Multiple Choice Questions (25) Short Answer Questions (11) Long Answer Questions (11)

CBSE Class 10 Science previous year papers, with answers →

Questions · page 9 of 15

Q161 CBSE 2023

"In human beings, the sex of a newborn child depends on the father and not the mother." Justify this statement with the help of a flow diagram.

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Every egg carries X, but a sperm carries X or Y, so the father's sperm alone decides the child's sex.
A human female is $44 + X X$ and a human male is $44 + X Y$ . When gametes are formed the mother can produce only one kind of egg, $22 + X$ , because she has no $Y$ chromosome at all; the father produces two kinds of sperm in about equal numbers, $22 + X$ and $22 + Y$ . If an $X$ -bearing sperm fertilises the egg the zygote is $44 + X X$ and the child is a girl; if a $Y$ -bearing sperm fertilises it the zygote is $44 + X Y$ and the child is a boy. The mother gives an $X$ to every child, so her contribution is the same either way and cannot decide the sex. Only the father's contribution varies, so the sex of the newborn child depends on the father and not on the mother.

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Q162 CBSE 2023
In order to trace the inheritance of traits Mendel crossed pea plants having one contrasting character or a pair of contrasting characters. When he crossed pea plants having round and yellow seeds with pea plants having wrinkled and green seeds, he observed that no plants with wrinkled and green seeds were obtained in the $F_{1}$ generation. When the $F_{1}$ generation pea plants were cross-bred by self-pollination, the $F_{2}$ generation had seeds with different combinations of shape and colour also.
1. (a)Write any two pairs of contrasting characteristics of pea plant used by Mendel other than those mentioned above.[1]
2. (b)Differentiate between dominant and recessive traits.[1]
3. (c)(i)State the ratio of the combinations observed in the seeds of $F_{2}$ generation (in the above case). What do you interpret from this result ?[2]
4. (c)(ii)OR : Given below is a cross between a pure violet flowered pea plant (V) and a pure white flowered pea plant (v). Diagrammatically explain what type of progeny is obtained in $F_{1}$ generation and $F_{2}$ generation : Pure violet flowered plant $(V V)$ $\times$ Pure white flowered plant $(v v)$ [2]
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Height and flower colour; dominant shows with one factor; $9 : 3 : 3 : 1$ shows independent inheritance; $V V \times v v$ gives $F_{2}$ in the ratio $3 : 1$ .
(a) Any two of the other pairs Mendel used, for example plant height, tall and dwarf, and flower colour, violet and white. (Pod shape inflated and constricted, pod colour green and yellow, and flower position axial and terminal are equally acceptable.) (b) A dominant trait appears in the $F_{1}$ generation and shows itself even when only one of its factors is present, as in $R r$ for round seeds. A recessive trait does not appear in $F_{1}$ and reappears in $F_{2}$ ; it shows itself only when both factors are recessive, as in $r r$ for wrinkled seeds. (c)(i) The ratio is $9 : 3 : 3 : 1$ — nine round and yellow, three round and green, three wrinkled and yellow and one wrinkled and green. Two of these, round green and wrinkled yellow, are combinations that neither parent had, so seed shape and seed colour are inherited independently of each other. OR (c)(ii) $V V$ gives only $V$ gametes and $v v$ only $v$ gametes, so every $F_{1}$ plant is $V v$ and all of them bear violet flowers, white being hidden. Self-pollinating $F_{1}$ ( $V v \times V v$ ) gives $V V$ , $V v$ , $V v$ and $v v$ , so the $F_{2}$ generation is $3$ violet flowered to $1$ white flowered, with the genotypes $1 V V : 2 V v : 1 v v$ .

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Q163 CBSE 2023
The most obvious outcome of the reproductive process is the generation of individuals of similar design, but in sexual reproduction they may not be exactly alike. The resemblances as well as differences are marked. The rules of heredity determine the process by which traits and characteristics are reliably inherited. Many experiments have been done to study the rules of inheritance.
1. (a)Why an offspring of human being is not a true copy of his parents in sexual reproduction ?[1]
2. (b)While performing experiments on inheritance in plants, what is the difference between $F_{1}$ and $F_{2}$ generation ?[1]
3. (c)(i)Why do we say that variations are useful for the survival of a species over time ?[2]
4. (c)(ii)OR : Study Mendel's cross between two plants with a pair of contrasting characters. RRYY (Round Yellow) × rryy (Wrinkled Green) He observed 4 types of combinations in $F_{2}$ generation. Which of these were new combinations ? Why do new features which are not present in the parents, appear in $F_{2}$ generation ?[2]
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Offspring get half their genes from each parent; $F_{1}$ is uniform, $F_{2}$ is not; variation helps survival; round green and wrinkled yellow are new.
(a) Because in sexual reproduction the offspring is formed by the fusion of two gametes and so receives half its genes from each parent, giving it a combination neither parent had; small inaccuracies in DNA copying and the shuffling of chromosomes during gamete formation add further variations. It therefore resembles both parents but copies neither. (b) The $F_{1}$ generation is obtained by crossing the two parents; all its members are hybrids, look alike and show only the dominant character, the recessive one being present but hidden. The $F_{2}$ generation is obtained by self-pollinating the $F_{1}$ ; it is not uniform, the recessive character reappears, and the characters occur in a definite ratio ( $3 : 1$ for one pair of contrasting characters, $9 : 3 : 3 : 1$ for two pairs). (c)(i) Environments change -- heat, cold, drought, a new predator or disease. A population in which every individual is alike would be destroyed entirely by a change none of them could tolerate, whereas in a varied population some individuals carry a feature suited to the new conditions, survive, reproduce and pass it on. Variation is thus the raw material of natural selection and the basis of a species surviving and adapting over time. OR (c)(ii) The new combinations are round green and wrinkled yellow; round yellow and wrinkled green are the parental types. They appear because the gene for seed shape is inherited independently of the gene for seed colour: the $F_{1}$ plant $R r Y y$ forms four kinds of gamete -- $R Y$ , $R y$ , $r Y$ and $r y$ -- and their random fusion brings together pairings that were never present in either parent, giving the $9 : 3 : 3 : 1$ ratio in $F_{2}$ .

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Q164 CBSE 2023
In some families, either rural or urban, females are tortured for giving birth to a female child. They do not seem to understand the scientific reason behind the birth of a boy or a girl. In fact the mother is not responsible for the sex of the child and it has been genetically proved that the sex of a newborn is determined by what the child inherits from the father.
1. (a)State the basis on which the sex of a newborn baby is determined in humans.[1]
2. (b)Why is the pair of sex chromosomes called a mismatched pair in males ?[1]
3. (c)(i)How is the original number of chromosomes present in the parents restored in the progeny ?[2]
4. (c)(ii)OR : Explain by giving two examples of the organisms in which the sex is not genetically determined.[2]
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Sex is fixed by the sex chromosome (X or Y) the child gets from the father; meiosis and fertilisation restore 46 chromosomes.
(a) Sex is determined by the pair of sex chromosomes, and specifically by the sex chromosome the child receives from its **father**. A human female is XX and a male is XY, so all eggs carry an X while half the sperms carry X and half carry Y. A child receiving X from the father is XX, a girl; a child receiving Y from the father is XY, a boy. The expected ratio is $1 : 1$ . (b) In a female both partners are X chromosomes - the same size and shape, carrying the same genes - so the pair is matched. In a male one partner is a normal-sized X and the other is a much shorter Y carrying far fewer genes, so the two do not match each other; hence XY is called a mismatched pair. (c)(i) Gametes are formed by meiosis, which halves the chromosome number, so each sperm and each egg carries $23$ chromosomes instead of $46$ . At fertilisation the two haploid gametes fuse and their sets combine, giving the zygote $23 + 23 = 46$ chromosomes - the parental number restored. Mitosis then copies this number into every body cell of the progeny. OR (c)(ii) In some animals the sex is decided by the environment and not by the genes. (1) In turtles (and crocodiles and some lizards) the temperature at which the fertilised eggs are incubated determines whether the hatchlings are male or female. (2) In snails an individual can change its sex during its lifetime, showing that sex is not fixed genetically.

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Q165 CBSE 2023
All human chromosomes are not paired. Most human chromosomes have a maternal and a paternal copy, and we have 22 such pairs. But one pair called the sex chromosomes, is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes. But men have a mismatched pair in which one is normal sized while the other is a short one.
1. (a)In humans, how many chromosomes are present in a Zygote and in each gamete ?
2. (b)A few reptiles rely entirely on environmental cues for sex determination. Comment.
3. (c)(i)"The sex of a child is a matter of chance and none of the parents are considered to be responsible for it." Justify it through flow chart only.
4. (c)(ii)OR : Why do all the gametes formed in human females have an X chromosome ?
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Zygote 46 and gamete 23 chromosomes; some reptiles use incubation temperature; every egg carries X, so the father's sperm decides sex.
(a) A zygote has $46$ chromosomes ( $23$ pairs — the $22$ matched pairs of the passage plus the sex chromosome pair), and each gamete has $23$ chromosomes, one from each pair. (b) In a few reptiles such as turtles, crocodiles and some lizards, sex is not decided by sex chromosomes but by the temperature at which the fertilised eggs are incubated: one range of temperature produces males and another produces females. This is environmental (temperature-dependent) sex determination, and it shows that a genetic switch is only one of the ways sex can be settled. (c)(i) Flow chart: Mother $X X$ and Father $X Y$ $\to$ meiosis $\to$ all eggs carry $X$ , while half the sperms carry $X$ and half carry $Y$ $\to$ egg $X$ + sperm $X$ gives $X X$ , a girl ( $50%$ ), and egg $X$ + sperm $Y$ gives $X Y$ , a boy ( $50%$ ). The mother can contribute only an $X$ , so she cannot decide the sex; the father makes both kinds of sperm in equal numbers and cannot choose which one fertilises the egg. The sex of the child is therefore purely a matter of chance and neither parent is responsible for it. (c)(ii) OR : Because a human female has a perfect pair of sex chromosomes, $X X$ . In meiosis the pair separates and each egg receives one member of it, and both members are $X$ — so every egg formed carries an $X$ chromosome.

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Q166 CBSE 2022

How is the number of chromosomes in the progeny of sexually reproducing organisms re-established ? Explain.

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Germ cells receive half the chromosomes by meiosis; fertilisation joins the two halves and re-establishes the full number ( $23 + 23 = 46$ ).
In sexually reproducing organisms such as human beings, the body cells carry two sets of chromosomes, one chromosome of each pair from each parent; the nucleus of a human body cell normally has 46 chromosomes in 23 pairs. The germ cells are formed through a special reduction division, meiosis, in which each germ cell receives only one chromosome of every pair - that is, half the number of chromosomes (23 in a human sperm or egg). At fertilisation a male germ cell fuses with a female germ cell, and the zygote receives one set from each, so the normal number is re-established ( $23 + 23 = 46$ ). The zygote then divides repeatedly by mitosis, which copies every chromosome, so the body cells of the progeny carry the same number as the parents. Halving the number in the germ cells and restoring it at fertilisation keeps the chromosome number, and the DNA content, of the species constant from one generation to the next.

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Q167 CBSE 2022

Using height (tallness / dwarfness) of a plant as an example, show that genes control the characteristics or traits in an organism.

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A normal gene gives an efficient enzyme, much hormone and a tall plant; an altered gene gives a short plant.
A gene is a section of DNA that provides the information for making a protein, and many proteins are enzymes. Plants have hormones that trigger growth, so the height of a plant depends on the amount of a particular plant hormone. The amount of hormone made depends on the efficiency of the process that makes it, which is carried out by an enzyme, and the enzyme is made from the information in a gene. If the gene for this enzyme is normal, the enzyme works efficiently, a lot of hormone is made and the plant is tall. If the gene has an alteration that makes the enzyme less efficient, less hormone is made and the plant is short (dwarf). So a change in the gene changes the enzyme, the amount of hormone and the height: genes control the characteristics or traits of an organism.

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Q168 CBSE 2022

In a cross between red coloured and white coloured flowers, when plants with red coloured flowers of F1 generation were self pollinated, plants of F2 generation were obtained in which 75% of plants were with red flowers and 25% plants were with white flowers. Explain the inheritance of traits in the above cross with the help of a flow chart only along with the ratio of plants obtained.

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Red RR × white rr gives all red Rr in $F_{1}$ ; selfing gives RR, Rr, Rr, rr, so $F_{2}$ has 3 red : 1 white.
Let $R$ stand for the form of the gene for red flowers (dominant) and $r$ for the form for white flowers (recessive). Parents: $R R$ (red) $\times$ $r r$ (white) Gametes: $R$ and $r$ $F_{1}$ : $R r$ - all plants have red flowers $F_{1}$ self-pollinated: $R r \times R r$ Gametes: $R$ and $r$ from each parent $F_{2}$ : $R R$ , $R r$ , $R r$ , $r r$ Ratio of plants in $F_{2}$ : 3 red : 1 white (75% red, 25% white); genotype ratio $1 R R : 2 R r : 1 r r$ . Red is dominant over white, so only red shows in $F_{1}$ ; white reappears in $F_{2}$ , in the $r r$ plants only.

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Q169 CBSE 2022

Pea plants were grown in two fields, X and Y. While field X produced pea plants with white flowers, field Y produced pea plants with blue flowers. In subsequent generations, it was observed that in the field X, the offsprings exhibited only the parental characters whereas in the field Y, majority of the offsprings exhibited variation in characters. State the possible reasons for these observations.

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Field X plants were pure-breeding and self-pollinated, giving no new gene combinations; field Y plants were cross-pollinated, giving new combinations and variation.
Field X: The white-flowered plants were most likely pure-breeding (true-breeding) and reproduced by self-pollination, the pollen and the egg coming from the same plant (the garden pea flower normally pollinates itself). So the offspring received the same set of genes as their parents, no new combinations of genes were formed, and they showed only the parental characters. (In Mendel's peas white is recessive, so white-flowered plants carry only the gene for white and breed true.) Field Y: The blue-flowered plants were most likely cross-pollinated, receiving pollen from other, genetically different plants, or were themselves hybrids. Sexual reproduction between different parents combines their DNA, so the offspring received new combinations of genes, and most of them showed variation in their characters.

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Q170 CBSE 2022
1. (a)What is the meaning of the following statement ? "In human beings, women have a perfect pair of sex chromosomes but men have a mismatched pair."
2. (b)On the basis of above statement, how can you arrive at a conclusion that the mother is not responsible for the sex of a child ? Explain in brief.
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(a) Women have two alike sex chromosomes, men an unlike pair; (b) the mother always gives X, so the father's sperm decides the sex.
(a) The statement means that the two sex chromosomes of a woman are alike - both are X chromosomes (XX), a perfect pair - whereas a man has two unlike sex chromosomes, one normal-sized X and one short Y (XY), a mismatched pair. (b) When gametes are formed, each egg or sperm receives only one chromosome of the pair. A woman (XX) can make only X-bearing eggs, so every child, girl or boy, receives an X from the mother. A man (XY) makes two kinds of sperm, half carrying X and half carrying Y. If a sperm carrying X fertilises the egg, the child is XX, a girl; if a sperm carrying Y fertilises it, the child is XY, a boy. Since the mother's contribution is always the same X, the sex of the child is decided by the chromosome in the father's sperm, so the mother is not responsible for the sex of the child.

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Q171 CBSE 2022

What is variation ? List two main reasons that may lead to variation in a population.

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Variation is the difference in characters among individuals of a species; it arises from DNA copying errors and sexual reproduction.
Variation is the occurrence of differences in characters (traits) among the individuals of a species, including the differences between offspring and their parents. Two main reasons that lead to variation in a population: 1. Errors in DNA copying: the copying of DNA during reproduction is not absolutely accurate, so the copies are similar to the original but may not be identical, and the changes are passed on. 2. Sexual reproduction: it combines the DNA of two parents, so each offspring receives a new combination of characters.

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Q172 CBSE 2022

“Sex chromosomes in human males and females are XY and XX respectively. Statistical probability of getting either a male or a female child is 50%.” Justify this statement giving reason.

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Every egg carries X while sperms are half X and half Y, so XX : XY = 1 : 1 and each sex has 50% probability.
A woman has two X chromosomes (XX), so every egg she produces carries an X chromosome. A man has one X and one Y chromosome (XY), so half of his sperms carry an X chromosome and half carry a Y chromosome. At fertilisation the egg is equally likely to be fertilised by either kind of sperm. If an X-carrying sperm fertilises the egg, the zygote is XX and develops into a girl; if a Y-carrying sperm fertilises it, the zygote is XY and develops into a boy. Crossing XX with XY therefore gives XX : XY = $1 : 1$ , so in each pregnancy the probability of a girl and of a boy is $\frac{1}{2}$ each, i.e. $50%$ . The sex of the child is decided by the sperm, that is, by what the child inherits from the father.

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Q173 CBSE 2022
1. (i)In a cross between violet flowered plants and white flowered plants, state the characteristics of the plants obtained in the $F_{1}$ progeny.
2. (ii)If the plants of $F_{1}$ progeny are self-pollinated, then what would be observed in the plants of $F_{2}$ progeny ?
3. (iii)If 100 plants are produced in $F_{2}$ progeny, then how many plants will show the recessive trait ?
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All $F_{1}$ plants are violet (Vv); $F_{2}$ gives violet : white = 3 : 1; 25 of 100 plants show the recessive white trait.
(i) All the $F_{1}$ plants have violet flowers; no white-flowered plant appears. Violet is the dominant trait. Each $F_{1}$ plant is a hybrid, $V v$ : it carries the factor for white flowers but does not show it. (ii) In the $F_{2}$ progeny both violet-flowered and white-flowered plants appear, in the ratio 3 violet : 1 white; the white (recessive) trait, hidden in $F_{1}$ , reappears. The genotypes are in the ratio $1 V V : 2 V v : 1 v v$ . (iii) One quarter of the $F_{2}$ plants show the recessive trait: $\frac{1}{4} \times 100 = 25$ plants, with white flowers.

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Q174 CBSE 2022

If we cross a tall plant (Tt) with a short plant (tt), what ratio of (Tt) and (tt) plants shall we obtain in $F_{2}$ generation ? Show with the help of a diagram (flow chart) only.

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Tt × tt gives Tt (tall) and tt (short) plants in the ratio 1 : 1.
Tt : tt = $1 : 1$ . Flow chart: Parents - tall (Tt) × short (tt); Gametes - T and t from the tall plant, t only from the short plant; Progeny - Tt, Tt, tt, tt; so Tt (tall) : tt (short) = $2 : 2 = 1 : 1$ . Half the plants are tall (Tt) and half are short (tt). (The flow chart is drawn in the figure.)

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Q175 CBSE 2022

Mendel crossed two pea plants with visible contrasting characteristics and found that there were no half-way characteristics in the plants of $F_{1}$ progeny. Explain the reason for this observation of Mendel.

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Only the dominant factor is expressed in each $F_{1}$ plant and factors do not blend, so no half-way plants appear.
Each characteristic is controlled by a pair of factors (genes), one inherited from each parent. When Mendel crossed a pure tall plant ( $T T$ ) with a pure short plant ( $tt$ ), every $F_{1}$ plant received one factor for tallness and one for shortness ( $T t$ ). Tallness is the dominant trait and shortness the recessive one, so in each $F_{1}$ plant only the dominant trait was expressed and all the plants were tall. The two factors do not blend, so no half-way (medium-height) plants appeared; the recessive factor was not lost either, because short plants reappeared in the $F_{2}$ generation in the ratio $3$ tall : $1$ short.

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Q176 CBSE 2022

Justify the statement “Sex of the children will be determined by what they inherit from their father”.

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The mother always gives X, while the father's sperm gives X or Y, so the child's sex is decided by the father's chromosome.
A woman has a perfect pair of sex chromosomes, $X X$ , and a man a mismatched pair, $X Y$ . So every egg carries an $X$ chromosome, while half the sperms carry an $X$ and half carry a $Y$ . If a sperm carrying $X$ fertilises the egg, the child is $X X$ , a girl; if a sperm carrying $Y$ fertilises it, the child is $X Y$ , a boy. Since the mother always contributes an $X$ , the sex of the child is decided by whether it inherits an $X$ or a $Y$ from its father, which justifies the statement.

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Q177 CBSE 2022
1. (a)Name the two types of gametes produced by men.
2. (b)Does a male child inherit X chromosome from his father ? Justify.
3. (c)How many types of gametes are produced by a human female ?
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Men form X and Y sperms; a son gets his X from his mother and Y from his father; women form only X eggs.
(a) Men produce two types of gametes (sperms): sperms carrying an X chromosome ( $22 + X$ ) and sperms carrying a Y chromosome ( $22 + Y$ ). (b) No. A male child (XY) inherits his Y chromosome from his father and his X chromosome from his mother. The mother (XX) can pass on only an X, so the son’s Y must come from the father’s Y-bearing sperm; had the father passed on his X, the child would be XX, a girl. (c) One type: every egg (ovum) of a human female carries an X chromosome ( $22 + X$ ).

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Q178 CBSE 2022

Write the total number of chromosomes along with the sex chromosomes that are present in a human male and a human female. Explain how, in sexually reproducing organisms, the number of chromosomes in the progeny remains the same as that of the parent.

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Male $44 + XY$ , female $44 + XX$ (46 each); meiosis halves the number in gametes and fertilisation restores 46.
A human male has $46$ chromosomes ( $23$ pairs) in his body cells: $44$ autosomes and the sex chromosomes X and Y, written $44 + XY$ . A human female also has $46$ chromosomes: $44$ autosomes and two X chromosomes, $44 + XX$ . In sexually reproducing organisms the gametes are formed by a special cell division (meiosis) in which the number of chromosomes is halved: each gamete receives only one chromosome of every pair (in humans $23$ : an egg carries $22 + X$ , a sperm $22 + X$ or $22 + Y$ ). At fertilisation a male and a female gamete fuse, so the zygote gets one set from each parent and has $23 + 23 = 46$ chromosomes, the same number as the parents. Halving at gamete formation and fusion at fertilisation together keep the chromosome number the same from one generation to the next.

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Q179 CBSE 2022
A cross was made between green-stemmed tomato plants denoted by (GG) and purple-stemmed tomato plants denoted as (gg) to obtain $F_{1}$ progeny.
1. (a)What colour of the stem would you expect in their $F_{1}$ progeny and why ?
2. (b)Give the percentage of purple-stemmed plants if $F_{1}$ plants are allowed to self-pollinate to produce $F_{2}$ progeny.
3. (c)Write the ratio between GG and gg plants in the $F_{2}$ progeny ?
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All $F_{1}$ plants are green ( $G g$ ) as $G$ is dominant; $25%$ of $F_{2}$ are purple, and $GG : g g = 1 : 1$ .
(a) All the $F_{1}$ plants will have green stems. Each receives G from the green parent and g from the purple parent, so each is Gg; G, the allele for green, is dominant (it is written with the capital letter), so one copy of it is enough for a green stem, and the purple trait stays hidden. (b) $25%$ . Self-pollinating Gg gives GG, Gg, Gg and gg; only gg, one in four, has a purple stem. (c) $GG : gg = 1 : 1$ (the $F_{2}$ genotypes are $1 GG : 2 Gg : 1 gg$ ).

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Q180 CBSE 2022
A red-eyed individual is crossed with a white-eyed individual to produce $F_{1}$ progeny with red eyes. When $F_{1}$ individuals are intercrossed, $F_{2}$ progeny is formed with both red as well as white-eyed individuals.
1. (a)How is the dominant trait identified ?
2. (b)What are recessive traits ?
3. (c)If 12 individuals are produced in $F_{2}$ generation, then how many white-eyed individuals would be obtained ? Calculate the ratio of red-eyed individuals to white-eyed individuals.
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Red is dominant; recessive traits stay hidden in $F_{1}$ ; 3 of the 12 are white-eyed, and red : white is $3 : 1$ .
(a) When two pure-breeding individuals with contrasting traits are crossed, the trait that alone appears in the $F_{1}$ progeny is the dominant trait. Here all the $F_{1}$ individuals have red eyes, so red eye colour is dominant. (b) Recessive traits are traits that remain hidden in the $F_{1}$ generation in the presence of the dominant factor and reappear in the $F_{2}$ generation; in Mendel's account they are expressed only when both copies of the factor are recessive, as in $r r$ . White eye colour is the recessive trait here. (c) The $F_{1}$ individuals are $R r$ , and $R r \times R r$ gives $1 R R : 2 R r : 1 r r$ , so one quarter of the $F_{2}$ is expected to be white-eyed: $\frac{1}{4} \times 12 = 3$ white-eyed individuals (and $9$ red-eyed). The ratio of red-eyed to white-eyed individuals is $9 : 3 = 3 : 1$ .

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