Detailed Notes -3- Chapter 4 Principles of Inheritance and Variation

4.6 SEX DETERMINATION

1. Observations in Insects:

   • In many insects, sex determination is of the XO type, where eggs carry an additional X chromosome besides autosomes.
   • Males have only one X chromosome, while females have a pair of X chromosomes.
   • Grasshoppers are an example of XO type sex determination.

2. XY Type Sex Determination:

   • In other insects and mammals, including humans, sex determination follows the XY type.
   • Both males and females have the same number of chromosomes, but males have one X and one Y chromosome, while females have a pair of X chromosomes.

3. Sex Chromosomes:

   • The X chromosome is designated as the sex chromosome, while all other chromosomes are called autosomes.
   • Males have autosomes plus XY chromosomes, while females have autosomes plus XX chromosomes.

4. Human and Drosophila Sex Chromosomes:

   • In humans and Drosophila, males have one X and one Y chromosome, while females have a pair of X chromosomes.

5. Male Heterogamety:

   • Both XO and XY types of sex determination exhibit male heterogamety.
   • Males produce two different types of gametes: some with an X chromosome and some without, or some with an X chromosome and some with a Y chromosome.

6. Female Heterogamety in Birds:

   • In birds, a different mechanism of sex determination is observed.
   • Females produce two different types of gametes in terms of sex chromosomes, exhibiting female heterogamety.
   • Female birds have one Z and one W chromosome, while males have a pair of Z chromosomes besides autosomes.

4.6.1 Sex Determination in Humans

1. XY Type Sex Determination:

   • In humans, the sex determining mechanism follows the XY type.
   • Out of the 23 pairs of chromosomes, 22 pairs are autosomes, which are the same in males and females.
   • The presence of an X and a Y chromosome determines the male characteristic, while females have a pair of X chromosomes.

2. Gamete Production:

   • During spermatogenesis in males, two types of gametes are produced: those with an X chromosome and those with a Y chromosome.
   • Females produce only one type of ovum, which carries an X chromosome.

3. Fertilization and Sex of Offspring:

   • When fertilization occurs, there is an equal probability of the ovum being fertilized by a sperm carrying either an X or a Y chromosome.
   • If the ovum is fertilized by an X-carrying sperm, the zygote develops into a female (XX), while fertilization by a Y-carrying sperm results in a male offspring.

4. Genetic Makeup Determines Sex:

   • The genetic makeup of the sperm, specifically whether it carries an X or a Y chromosome, determines the sex of the child.

5. Probability of Male or Female Offspring:

   • In each pregnancy, there is always a 50 per cent probability of either a male or a female child being born.

6. Social Implications:

   • Unfortunately, societal attitudes in some cultures may lead to blame or mistreatment of women for giving birth to female children, despite the biological fact that sex determination is determined by sperm genetics.

4.6.2 Sex Determination in Honey Bee

1. Honey Bee Sex Determination:

   • Sex determination in honey bees is based on the number of chromosome sets an individual receives.
   • Offspring formed from the union of a sperm and an egg develop as females (queens or workers), while unfertilized eggs develop as males (drones) through parthenogenesis.
   • Females are diploid, meaning they have two sets of chromosomes (32 chromosomes), while males are haploid, having only one set of chromosomes (16 chromosomes).
   • This system is called haplodiploid sex determination.

2. Special Features of Haplodiploid System:

   • In honey bees, males produce sperm by mitosis.
   • Males do not have fathers and thus cannot have sons but have grandfathers and can have grandsons.

3. Comparison with Birds:

   • In birds, the sex determination mechanism differs from that of honey bees.
   • In birds, it is either the sperm or the egg that determines the sex of the chicks.

4.7 MUTATION

1. Definition and Consequences:

   • Mutation refers to alterations in DNA sequences, leading to changes in both genotype and phenotype.
   • These alterations can occur through mechanisms like recombination and point mutations.
   • Mutations can result in abnormalities or aberrations, particularly in chromosomes, and are commonly observed in cancer cells.

2. Types of Mutations:

   • Point mutations involve changes in a single base pair of DNA. An example is sickle cell anemia.
   • Frame-shift mutations occur due to deletions or insertions of base pairs of DNA, causing a shift in the reading frame.

3. Mechanisms and Factors:

   • The mechanism of mutation is complex and involves various chemical and physical factors.
   • Factors that induce mutations are known as mutagens. UV radiation is one example of a mutagenic agent that can cause mutations in organisms.

4. Scope of Discussion:

   • While mutations and their mechanisms are important topics, detailed discussions about them may be beyond the scope of certain levels of study or discussions.

4.8 GENETIC DISORDERS

4.8.1 Pedigree Analysis

1. Pedigree Analysis:

   • Pedigree analysis involves studying the inheritance patterns of traits within families across multiple generations.
   • Since controlled crosses like those performed in pea plants are not feasible in humans, pedigree analysis provides an alternative method.
   • It helps in tracing the inheritance of specific traits, abnormalities, or diseases through family trees.
   • Standard symbols are used to represent individuals, relationships, and affected statuses in pedigree charts.

2. Genetic Basis:

   • Every feature in an organism is controlled by genes located on DNA, which is transmitted from one generation to the next without alteration.
   • Occasionally, changes or alterations in the genetic material occur, referred to as mutations.
   • These mutations can lead to genetic disorders or abnormalities in human beings.

4.8.2 Mendelian Disorders 

1. Classification:

   • Genetic disorders are broadly categorized into Mendelian disorders and Chromosomal disorders.
   • Mendelian disorders result from alterations or mutations in a single gene.
   • These disorders follow Mendelian principles of inheritance and can be traced through pedigree analysis.

2. Examples:

   • Common Mendelian disorders include Haemophilia, Cystic fibrosis, Sickle-cell anaemia, Colour blindness, Phenylketonuria, Thalassemia, etc.
   • Each of these disorders is caused by mutations in a specific gene, leading to characteristic symptoms and inheritance patterns.

3. Inheritance Patterns:

   • Mendelian disorders may be dominant or recessive, depending on whether the mutated gene is on one or both chromosomes of a gene pair.
   • Pedigree analysis helps in determining the pattern of inheritance for these disorders within families.
   • Some Mendelian disorders, like haemophilia, may be linked to the sex chromosome (X-linked), showing distinct inheritance patterns.

4. Pedigree Analysis:

   • Pedigree analysis allows researchers to understand whether a trait is dominant or recessive, and whether it is linked to autosomes or sex chromosomes.
   • Designing pedigrees for characters linked to both autosomes and sex chromosomes can provide further insights into inheritance patterns and genetic traits.

1. Colour Blindness:

   • Type: Sex-linked recessive disorder.
   • Cause: Mutation in genes on the X chromosome affecting red or green cone cells in the eyes.
   • Incidence: More common in males (8%) than females (0.4%).
   • Inheritance: Passed from carrier mothers to sons; daughters may be carriers if mother is a carrier and father is color blind.

2. Haemophilia:

   • Type: Sex-linked recessive disorder.
   • Cause: Mutation affecting a protein involved in blood clotting.
   • Symptoms: Excessive bleeding from minor injuries due to impaired clotting.
   • Inheritance: Passed from carrier females to sons; extremely rare for females to be affected unless both parents are carriers or affected.

3. Sickle-cell Anaemia:

   • Type: Autosomal recessive trait.
   • Cause: Mutation affecting haemoglobin molecule, causing sickle-shaped red blood cells under low oxygen conditions.
   • Inheritance: Requires both parents to be carriers; heterozygous individuals are carriers, homozygous individuals exhibit the diseased phenotype.

4. Phenylketonuria (PKU):

   • Type: Autosomal recessive trait.
   • Cause: Deficiency of an enzyme that converts phenylalanine to tyrosine, leading to accumulation of phenylalanine and mental retardation.
   • Inheritance: Requires both parents to be carriers; affected individuals lack the enzyme for phenylalanine metabolism.

5. Thalassemia:

   • Type: Autosomal recessive blood disorder.
   • Cause: Mutation affecting synthesis of one of the globin chains in haemoglobin.
   • Types: α-Thalassemia (affecting α-globin genes) and β-Thalassemia (affecting β-globin genes).
   • Inheritance: Requires both parents to be carriers; α-Thalassemia can result from mutation or deletion of α-globin genes, while β-Thalassemia results from mutation of β-globin genes.

4.8.3 Chromosomal Disorders

1. Aneuploidy:

   • Cause: Failure of segregation of chromatids during cell division, leading to gain or loss of a chromosome(s).
   • Examples: Down’s syndrome (extra copy of chromosome 21), Turner’s syndrome (loss of an X chromosome in females).

2. Polyploidy:

   • Cause: Failure of cytokinesis after telophase stage of cell division, resulting in an increase in a whole set of chromosomes.
   • Occurrence: Commonly seen in plants.

3. Down’s Syndrome:

   • Cause: Trisomy of chromosome 21 (presence of an extra copy).
   • Symptoms: Short stature, small round head, furrowed tongue, partially open mouth, broad palms with characteristic crease, developmental retardation.

4. Klinefelter’s Syndrome:

   • Cause: Presence of an additional X chromosome (karyotype 47, XXY).
   • Symptoms: Masculine development with some feminine traits (e.g., gynecomastia), sterility.

5. Turner’s Syndrome:

   • Cause: Absence of one X chromosome (karyotype 45, X0) in females.
   • Symptoms: Sterility, rudimentary ovaries, lack of secondary sexual characteristics.