15.1 X and Y Chromosomes 04 November 2024

15.1 X and Y Chromosomes

Overview of Mendelian Genetics

  • Mendel's experiments concluded that the identity of the parent (male or female) does not affect the progeny ratios from crosses, termed as reciprocal crosses.

  • Most of the time, reciprocal crosses yield similar progeny proportions, but there are exceptions.

Example: Red-Green Color Blindness

  • Red-green color blindness serves as an illustrative exception:

    • Cross between a colorblind man (X^c Y) and a normal vision woman (X^N X^N):

      • All offspring have normal vision (X^N Y and X^N X^N).

    • Reciprocal cross between a colorblind woman (X^c X^c) and a normal vision man (X^N Y):

      • All daughters have normal vision (X^N X^c).

      • All sons are colorblind (X^c Y).

Chromosomal Basis of Exceptions

  • Analysis of exceptions to Mendel's findings through:

    • Chromosome structure.

    • Inheritance strategies.

Chromosome Structure and Function

  • Chromosomes typically exist in pairs called homologous chromosomes, containing genes along their lengths:

    • One chromosome of each pair is inherited from each parent.

Sex Chromosomes

  • In many animals, sex is determined by specific pairs of unmatched chromosomes labeled as X and Y:

    • Female: XX (two X chromosomes)

    • Male: XY (one X and one Y chromosome)

  • The X chromosome:

    • Exceeds 150 megabases in size.

    • Gene-rich with over 1000 genes.

  • The Y chromosome:

    • Smaller, about 50 megabases.

    • Approximately 50 protein-coding genes.

Autosomes

  • Autosomes are the non-sex chromosomes:

    • Present in pairs in both males and females.

    • In humans, there are 22 pairs of autosomes.

    • Determine the majority of genetic traits unrelated to sex.

Homologous Regions and Meiosis

  • Regions of homology are critical for pairing during meiosis, allowing crossover events to occur.

  • Exceptions in species where X and Y chromosomes may be similar in size or shape, like some mosquitoes.

  • In grasshoppers:

    • No Y chromosome exists; females have two X chromosomes, while males have one.

Alternative Sex Chromosome Systems

  • In certain species (birds, moths, butterflies), the sex chromosome designation is reversed:

    • Males have two identical sex chromosomes.

    • Females possess two different ones.

Significance of Chromosomal Segregation

  • Mendel's failure to connect sex with trait inheritance led to missed insights on chromosome segregation mechanics:

    • Male meiosis results in an equal split of sperm with X and Y chromosomes.

    • Female meiosis results in eggs with one X chromosome, leading to combinatory fertilization outcomes.

Expected Sex Ratios

  • At conception, a 1:1 male to female ratio is expected:

    • Half of the fertilized eggs will be XX (female) and half XY (male).

  • The secondary sex ratio (observed at birth) typically shows:

    • About 100 females for every 105 males in the U.S.

  • Excess male births can be attributed to differences in embryo survival rates:

    • Female embryos may be less likely to survive from conception to birth.

    • Male mortality is higher from birth to reproductive age.

Survival and Mortality Patterns

  • By reproductive maturity, sex ratios approximate 1:1.

  • By the age of 85 and older, the ratio shifts to about 2:1 in favor of females due to male mortality rates.

Randomness of Sex Determination

  • Birth sexes appear to be random and don’t show tendencies towards specific distributions in families.

  • Families with skewed sex distribution could occur purely by chance and should not be mistaken as a trend.