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Principles of Inheritance and Variation - Sex Determination

Grade 12CBSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Sex determination is the biological process that determines the development of sexual characteristics in an organism, primarily governed by sex chromosomes (allosomes).

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Male Heterogamety: A system where the male produces two different types of gametes. Examples include the XYXY type (Humans, DrosophilaDrosophila) and the XOXO type (Grasshoppers).

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In XYXY type, males have one XX and one YY chromosome plus autosomes (AA), represented as AA+XYAA + XY. Females have two XX chromosomes, represented as AA+XXAA + XX.

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In XOXO type, males possess only one XX chromosome (AA+XOAA + XO), while females possess two (AA+XXAA + XX). The presence of only one XX determines the male sex.

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Female Heterogamety: A system where the female produces two different types of gametes. Example includes the ZWZW type found in birds. Females are AA+ZWAA + ZW and males are AA+ZZAA + ZZ.

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Haplodiploid Sex Determination: Observed in honey bees. Males (drones) develop from unfertilized eggs via parthenogenesis and are haploid (nn). Females (queens/workers) develop from fertilized eggs and are diploid (2n2n).

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In humans, the YY chromosome carries the SRYSRY (Sex-determining Region Y) gene, which triggers the development of male gonads. In the absence of YY, the embryo develops as a female.

📐Formulae

Human Female=44A+XX\text{Human Female} = 44A + XX

Human Male=44A+XY\text{Human Male} = 44A + XY

Bird Female=2A+ZW\text{Bird Female} = 2A + ZW

Bird Male=2A+ZZ\text{Bird Male} = 2A + ZZ

Honey Bee (Female)=2n=32\text{Honey Bee (Female)} = 2n = 32

Honey Bee (Male)=n=16\text{Honey Bee (Male)} = n = 16

Sex Ratio Probability=12(XX):12(XY)\text{Sex Ratio Probability} = \frac{1}{2} (XX) : \frac{1}{2} (XY)

💡Examples

Problem 1:

In a cross between a male grasshopper and a female grasshopper, determine the chromosomal composition of the offspring and the sex ratio.

Solution:

The male is XOXO and the female is XXXX. Gametes from male: 50%50\% carry XX, 50%50\% carry no sex chromosome (OO). Gametes from female: 100%100\% carry XX. Offspring: 50%50\% XXXX (Female) and 50%50\% XOXO (Male).

Explanation:

This is an example of male heterogamety where the sperm determines the sex of the offspring based on the presence or absence of the XX chromosome.

Problem 2:

If a honey bee drone has 1616 chromosomes, how many chromosomes will the worker bees and the queen have?

Solution:

Worker bees and queens are females, which are diploid (2n2n). Since the drone is haploid (n=16n = 16), the females will have 2n=2×16=322n = 2 \times 16 = 32 chromosomes.

Explanation:

Honey bees follow a haplodiploid system where males are produced from unfertilized eggs (nn) and females from fertilized eggs (2n2n).

Problem 3:

A couple has 3 daughters. What is the probability that their fourth child will be a son?

Solution:

The probability is 1/21/2 or 50%50\%.

Explanation:

In every pregnancy, the segregation of XX and YY chromosomes from the father occurs independently. The fusion of an XX-carrying egg with either an XX-carrying sperm (XXXX) or a YY-carrying sperm (XYXY) always has a theoretical ratio of 1:11:1 regardless of previous outcomes.