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Q11:
How is sex determined in human beings?
Solution :
Genetic Basis of Sex Determination in Humans
In humans, sex determination is governed by a chromosomal mechanism classified as the $XX-XY$ type. This system is primarily driven by the presence or absence of the $Y$ chromosome, which carries the male-determining genetic information.
Step 1: Chromosomal Constitution
A diploid human somatic cell contains $46$ chromosomes, organized into $23$ pairs. These are categorized into:
- Autosomes: $22$ pairs ($44$ chromosomes) that carry genes for somatic characteristics.
- Sex Chromosomes (Allosomes): $1$ pair ($2$ chromosomes) responsible for sexual dimorphism.
The chromosomal makeup is defined as:
- Females: $44 + XX$ (Homogametic; produce only one type of gamete: $22 + X$).
- Males: $44 + XY$ (Heterogametic; produce two types of gametes: $22 + X$ and $22 + Y$).
Step 2: The Mechanism of Gametogenesis
During meiosis, the homologous chromosome pairs segregate [Per Mendel’s Law of Segregation].
| Parent | Diploid Constitution | Gametes Produced |
|---|---|---|
| Female | $44 + XX$ | $22 + X$ (100%) |
| Male | $44 + XY$ | $22 + X$ (50%) and $22 + Y$ (50%) |
Step 3: Fertilization and Sex Ratio Probability
Sex determination is strictly determined by the male gamete (sperm) that fertilizes the ovum. Since the ovum always contributes an $X$ chromosome, the gender of the offspring depends on whether the sperm cell carries an $X$ or $Y$ chromosome.
Step 4: The Role of the SRY Gene
The determination of the male phenotype is specifically regulated by the SRY (Sex-determining Region Y) gene located on the short arm of the $Y$ chromosome [Per Molecular Genetics]. This gene triggers the differentiation of the undifferentiated gonads into testes, which subsequently secrete testosterone and Anti-Mullerian Hormone (AMH) to masculinize the fetus. In the absence of the $Y$ chromosome and the $SRY$ gene, the fetus develops into a female phenotype.
Final Solution: Sex in humans is determined by the chromosomal constitution of the sperm at the moment of fertilization. An $X$-bearing sperm fertilizing an $X$-bearing ovum results in a female ($44 + XX$), while a $Y$-bearing sperm fertilizing an $X$-bearing ovum results in a male ($44 + XY$). Statistically, there is a $50\%$ probability for each sex per fertilization event.
More Questions from Class 12 Biology Principles of Inheritance and Variation EXERCISES
- Q1: Mention the advantages of selecting pea plant for experiment by Mendel.
- Q1: Mention the advantages of selecting pea plant for experiment by Mendel.
- Q2(a): Differentiate between the following – (a) Dominance and Recessive
- Q2(a): Differentiate between the following – (a) Dominance and Recessive
- Q2(b): Differentiate between the following – (b) Homozygous and Heterozygous
- Q2(b): Differentiate between the following – (b) Homozygous and Heterozygous
- Q2(c): Differentiate between the following – (c) Monohybrid and Dihybrid.
- Q2(c): Differentiate between the following – (c) Monohybrid and Dihybrid.
- Q3: A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
- Q3: A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
- Q4: Explain the Law of Dominance using a monohybrid cross.
- Q4: Explain the Law of Dominance using a monohybrid cross.
- Q5: Define and design a test-cross.
- Q5: Define and design a test-cross.
- Q6: Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.
- Q6: Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.
- Q7(a): When a cross in made between tall plant with yellow seeds (\(TtYy\)) and tall plant with green seed (\(Ttyy\)), what proportions of phenotype in the offspring could be expected to be (a) tall and green.
- Q7(a): When a cross in made between tall plant with yellow seeds ((TtYy)) and tall plant with green seed ((Ttyy)), what proportions of phenotype in the offspring could be expected to be (a) tall and green.
- Q7(b): When a cross in made between tall plant with yellow seeds (\(TtYy\)) and tall plant with green seed (\(Ttyy\)), what proportions of phenotype in the offspring could be expected to be (b) dwarf and green.
- Q7(b): When a cross in made between tall plant with yellow seeds ((TtYy)) and tall plant with green seed ((Ttyy)), what proportions of phenotype in the offspring could be expected to be (b) dwarf and green.
- Q8: Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in \(F_{1}\) generation for a dibybrid cross?
- Q8: Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in (F_{1}) generation for a dibybrid cross?
- Q9: Briefly mention the contribution of T.H. Morgan in genetics.
- Q9: Briefly mention the contribution of T.H. Morgan in genetics.
- Q10: What is pedigree analysis? Suggest how such an analysis, can be useful.
- Q10: What is pedigree analysis? Suggest how such an analysis, can be useful.
- Q11: How is sex determined in human beings?
- Q12: A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.
- Q12: A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.
- Q13(a): Explain the following terms with example (a) Co-dominance
- Q13(a): Explain the following terms with example (a) Co-dominance
- Q13(b): Explain the following terms with example (b) Incomplete dominance
- Q13(b): Explain the following terms with example (b) Incomplete dominance
- Q14: What is point mutation? Give one example.
- Q14: What is point mutation? Give one example.
- Q15: Who had proposed the chromosomal theory of the inheritance?
- Q15: Who had proposed the chromosomal theory of the inheritance?
- Q16: Mention any two autosomal genetic disorders with their symptoms.
- Q16: Mention any two autosomal genetic disorders with their symptoms.
CBSE Solutions for Class 12 Biology Principles of Inheritance and Variation
Chapters in CBSE - Class 12 Biology
- Reproduction in Organisms
- Human Reproduction
- Reproductive Health
- Molecular Basis of Inheritance
- Evolution
- Human Health and Disease
- Strategies for Enhancement in Food Production
- Microbes in Human Welfare
- Biotechnology: Principles and Processes
- Biotechnology and Its Applications
- Organisms and Populations
- Ecosystem
- Biodiversity and Conservation
- Environmental Issues
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