Unit 3: Sex Determination and Inheritance

1. Mechanisms of Sex Determination

Sex determination refers to the biological system that determines the development of sexual characteristics in an organism.

In many organisms, sex is determined by specific sex chromosomes:

XX-XY System: Found in mammals (including humans) and Drosophila. Females are homogametic (XX) and males are heterogametic (XY).
ZZ-ZW System: Found in birds and some reptiles. Males are homogametic (ZZ) and females are heterogametic (ZW).
XX-XO System: Found in some insects like grasshoppers. Females are XX and males have only one X chromosome (XO).

Cytoplasmic/Genic: Certain cytoplasmic factors or specific genes on autosomes can influence sex.
Hormonal: In some animals, hormones produced during development can shift sexual differentiation (e.g., Free-martin in cattle).
Environmental: External factors like temperature determine sex in some reptiles (e.g., crocodiles and turtles).

Sex-linked inheritance involves the transmission of genes located on the sex chromosomes.

X-linked Inheritance: Genes located on the X-chromosome. Since males have only one X, recessive traits appear more frequently in males (e.g., Color blindness, Hemophilia).
Y-linked (Holandric) Inheritance: Genes located only on the Y-chromosome, passed strictly from father to son (e.g., TDF gene, hypertrichosis of the ear).
Criss-cross Inheritance: A characteristic of X-linked traits where the trait passes from a father to his grandson through his daughter.

Definition: The transmission of genes that occur outside the nucleus, specifically in organelles like mitochondria and chloroplasts.

Key characteristics of extra-chromosomal inheritance:

Uniparental/Maternal Inheritance: Since the zygote receives most of its cytoplasm from the egg, these traits are usually inherited from the mother.
Lack of Mendelian Ratios: These traits do not follow standard Mendelian segregation patterns.
Example: Mitochondrial DNA (mtDNA) mutations in humans or plastid inheritance in plants.

Identify the System: When solving genetics problems, check if the male or female is heterogametic (XY vs ZW).
Maternal Link: If a trait is passed from a mother to all her children but a father never passes it, suspect extra-chromosomal/mitochondrial inheritance.
Hemizygous: Remember that human males are hemizygous for X-linked genes because they have only one X chromosome.

Confusing Sex-linked with Sex-limited traits. Sex-linked genes are on sex chromosomes; sex-limited genes are often on autosomes but only expressed in one sex (e.g., milk production).
Assuming all sex is determined by chromosomes. Always mention environmental factors like temperature for reptiles in your answers.