Sex Determination and Sex Linkage PDF

SEX DETERMINATION AND SEX LINKAGE [email protected] 1  Objectives: To analyze the causes of sex determination in various organisms 2 Sex Determination Most mechanisms for th...

SEX DETERMINATION AND SEX LINKAGE [email protected] 1  Objectives: To analyze the causes of sex determination in various organisms 2 Sex Determination Most mechanisms for the determination of sex are under genetic (and hormonal) control. Basically, four types of chromosomal sex-determining mechanisms exist. These are the XY, ZW, XO, and compound chromosomal mechanisms. In humans, and most other mammals, there are two different sex chromosomes (i.e., X and Y chromosomes); heteromorphic (a pair of chromosomes that differ in size, shape) 3  The presence of the Y chromosome determines maleness. Normal males have 22 pairs of autosomes and an X and a Y chromosome. Since the male produces two kinds of gametes (X and Y) as far as the sex chromosomes are concerned, he is said to be the heterogametic sex. Females also have 22 pairs of autosomes but have two X chromosomes. The female, producing only one kind of gamete, is the homogametic sex. 4 XY method of sex determination 5  In both Drosophila and humans, normal females and males have an XX and an XY sex chromosome composition respectively. Still, one should not assume that, in both species, the genes for females are on the X chromosomes and those for males are on the Y. The Y chromosome plays no significant role in sex determination in Drosophila. Rather, it contains genes active in forming sperm in adults 6 The SRY (Sex-determining region Y) gene A gene called SRY has been identified on the short arm of the Y chromosome. It encodes a gene product called testis-determining factor (TDF). SRY is highly conserved in mammals. The gene in combination with several other genes, encodes a DNA-binding protein that activates the expression of testicular development and fertility. 7 Heterogametic females (ZZ-ZW) This method of sex determination is found in many species, including most birds, butterflies, moths, and some fish. The mode is basically identical to the XX-XY mechanism. Females are heterogametic (designated as ZW) whilst males are homogametic (ZZ). 8 Sex Linkage ✓ those inheritance patterns of genes located on the sex chromosomes (X and Y). ✓ Sex linkage applies to genes that are located on the sex chromosomes. ✓ They are considered sex-linked because their expression and inheritance patterns differ between males and females. ✓ In humans, a few genes are known to reside in the non-homologous portion of the Y chromosome. ✓ In such cases, the trait would be expressed only in males and would always be transmitted from father to son. ✓ Such completely Y-linked genes are called holandric genes. 9  Since males only have one X chromosome, X-linked recessive disorders are more common in males. Females have two X chromosomes, so X-linked recessive disorders are rarer in females. NOTE Genes located on the X chromosome are called X-linked genes. Genes located on the Y chromosome are called Y-linked genes. X-linked genes exhibit different inheritance patterns compared to autosomal genes. Many X-linked genes are involved in conditions like color blindness, haemophilia, and Duchenne muscular dystrophy 10 X-Linked Disorders ✓ X-linked disorders are genetic conditions that arise from mutations in genes on the X chromosome. ✓ These disorders exhibit unique inheritance patterns due to the presence of one X chromosome in males (XY) and two in females (XX). 11 Types of X-Linked Disorders 1. X-Linked Recessive Disorders Inheritance Pattern ✓ More males are affected than females. ✓ Affected males cannot pass the disorder to their sons, but all daughters will be carriers. ✓ Carrier females have a 50% chance of passing the mutation to their offspring. 12 Examples Hemophilia ✓ A disorder that impairs the blood's ability to clot. Symptoms: Prolonged bleeding, spontaneous bleeding, joint pain, and swelling. 13 Duchenne Muscular Dystrophy (DMD) ✓ A progressive muscle degeneration and weakness disorder. Symptoms: Muscle weakness, difficulty walking, respiratory difficulties, and cardiac issues. 14 Colour Blindness ✓ An inability to distinguish certain colors. Symptoms ✓ Difficulty distinguishing between red and green hues (most common form). 15 2. X-Linked Dominant Disorders Inheritance Pattern ✓ Both males and females can be affected. ✓ Affected males pass the disorder to all daughters but not sons. ✓ Affected females have a 50% chance of passing the disorder to their offspring. 16 Examples Rett Syndrome ✓ A neurodevelopmental disorder primarily affecting females. ✓ Symptoms: Loss of motor skills, speech difficulties, repetitive hand movements, and intellectual disability. 17 Fragile X Syndrome ✓ A genetic condition causing intellectual disability, behavioral challenges, and distinctive physical features. Symptoms: Learning disabilities, developmental delays, social and behavioral issues. 18 Additional Reading: Read on the following; H. Henking C. E. McClung E. B. Wilson Note: They all contributed to investigations relating chromosomes to sex determination. 19