Chapter 7 Sex and inheritance 7.1 Sex determination 7.2 Sex-linked inheritance

Sex chromosome Autosomes Primary sexual characteristics Secondary sexual characteristics Incubate Ovary Testis Hermaphrodite Homogametic Heterogametic Testosterone Pseudoautosomal region

7.1 Sex determination ():

Primary sex determination relates to the formation of ovaries or testes. This can be regulated by genes and/or environmental factors. Secondary sexual characteristics are also under genetic regulation.

In alligators () sex is determined by the temperature at which eggs are incubated.

Many species (plant and invertebrate) are hermaphrodite.

sex-chromosomesautosomesHeterogametic: - X Y

e.g. female 2n=24 (11A)+XX male 2n= 23 (11A)+X0

7.1.1 Sex chromosome systems (): Three different sex chromosome systems have been described:

XX-XO This is found in many insect species.

maleXO femaleXX

normal human male 22A+XY female 22A+XXHuman sex determination and differentiation sperm type 22A+X ( 22A+X; 22A+Y) Y - male XY-; XXY,XXXY,XXXXY- XX-; XO- XX-XY This is found in mammals and in certain insects including Drosophila.

Sex determination in Drosophila ( XY):

ZW-ZZ This is found in birds, snakes, where the female is ZW and the male ZZ.

7.1.2 (multiple type) 2n=32 n=16

7.1.3 Environment and sex differentiationnutrition >5d 2~3d (infertile)2) temperature (XY) 20 half male and half female >30 male, reverse sex from female

Y -- H-Y X -- 7.1.4 H-Y antigen and sex -

7.2 Sex-linked inheritance ()

7.2.1 Morgan et al. found in 1910Morgan chose red-eyed female Drosophila mate with white-eyed male, F1 generation had red eyes. If using white female mate with red male, F1 just had the female with red eyes.PF1

White color is regulated by X linked recessive mutant gene carried by X chromosome , stated as Xw X+X+ XwY Allele X+ Xw YF1 X+ Xw X+Y 1:1 W,,.F2 X+ Xw X+Y X+ Y X+ X+ X+ X+ Y Xw X+ Xw XwY 3 : 1 ,;

When reverse cross happened: XwXw X+Y Allele Xw X+ YF1 X+ Xw XwY 1:1 WF2 X+ Xw XwY Xw Y X+ X+ Xw X+ Y Xw Xw Xw XwY 1 : 1 ,

7.2.2 (ZBW) (ZbZb)F1 ____ ------- (ZbW) (ZBZB)F1 -------- regulated by Z linked dominant gene B Z F1 F2 F1F2

7.2.3 A useful example is colorblindness in humans

This is due to a recessive allele of a gene which maps to the X chromosome. The normal allele is denoted B and the mutant allele is b. A colorblind man must have inherited the b allele with his X chromosome from his mother. If she had normal vision then she must have been a heterozygote Bb. A colorblind man cannot transmit his b allele to his son as, by definition, his son must inherit a Y chromosome from his father. By the same rule he must pass b to all of his daughters. Any daughter that inherits the b allele from her father and a normal allele from her mother will be a carrier and transmit the syndrome to, on average, half of her sons.

If a female-carrier married a normal man X+Xc X+Y X+ Y X+ X+ X+ X+Y Xc X+ Xc XcY A female-carrier married a male colorblindness X+Xc XcY X c Y X+ X+ X c X+Y X c X c X c X cY

7.2.4 Hemophilia in humans Hemophilia is a sex linked trait in humans, inherited in the same way with white eye color in Drosophila. Males are hemizygous, receiving their only X chromosome from their mother. Females are homozygous, inheriting X chromosomes from both parents. If a female has a defective gene on one of her two X chromosomes, she will be protected from its effects by the normal gene on her second X chromosome. If a male has a mutant X and a normal Y chromosome, he will be affected by a X-linked disease.

A son, whose mother has two normal alleles, will not be affected by hemophilia even if the father has the disease and the defective gene. A daughter of the same parents will be a heterozygous carrier. A heterozygous carrier mother and a normal father pass the gene for hemophilia on to possibly one-half of their children. Half the daughters will be carriers and half the sons will be hemophilic. The rest of the siblings will be normal.

Daughters, as long as one of her parents is genotypically normal, can only be carriers. The normal gene on the second X chromosome counteracts the defect and the daughters do not suffer from the trait. If a son receives the defective gene from his mother, he will be hemophilic because the Y chromosome can not counteract the defective gene located on his X chromosome.

In the case of hemophilia, family records show that Queen Victoria had a mutant allele for hemophilia and transmitted the disease to some of her sons and many of the European royal houses through marriage of her daughters. As there is no evidence of hemophilia in her ancestors, Queen Victoria must have inherited a mutation that arose in the germ cells of one of her parents.

Sex-linked inheritance - Recessive alleles of genes mapping to the X chromosome are not expressed in heterozygous female mammals but will be expressed in male because males have only one X chromosome. Males transmit the recessive allele to their daughters, where it is not expressed. They are referred to as carrier females. The daughters, in turn, transmit the allele to half of their sons, where it is re-expressed.

7.2.5 Holandric inheritance - Some genes are on the Y chromosome and are passed directly from father to son. This is known as holandric inheritance. - Y A gene for hairs on the outer rim of the ear appears to show this pattern of holandric inheritance. Father son just for man

7.2.6 Sexinfluenced inheritance -

e.g. H h female male HH Hh hh HH

A small region of homology exists between the X and Y chromosomes. Genes in this region, the pseudo-autosomal region, do not show sex-linked inheritance.