The Genetic Determination of Sex

Homologous Chromosomes As a general rule homologous chromosomes are the same size. The exception to this rule is the sex chromosomes. The cells of the females of most species contain 2 identical sex chromosomes called X chromosomes.

Homologous Chromosomes In males, the 2 sex chromosomes are the X chromosome and the smaller Y chromosome. The large X chromosome carries many genes, while the Y chromosome carries only a few – mainly for maleness.

SEX As the females chromosomes are the same, females are called the Homogametic Sex. Males, with 2 different sex chromosomes, are called the Heterogametic Sex. This is true for humans, fruit flies and many other organisms.

Exceptions In birds, butterflies and moths, the female is heterogametic and the male homogametic. The sex chromosome symbols are sometimes changed to Z=X and W=Y; so male birds are ZZ and females are ZW In some insects such as grasshoppers, there is no Y chromosome at all, so: XO = male and XX = female.

Exceptions In Hymenoptera (ants, bees and wasps) no sex chromosomes have been identified. The fertile queen can lay fertilised eggs (2n) to produce females or unfertilised eggs (n) to produce males.

Exceptions In some reptiles and some marine fish, sex is determined by environment. In reptiles the sex can be determined by the temperature of the eggs as they develop. Cooler temps produce females, higher temps produce males. Some marine fish have the ability to change sex if the need arises.

The Barr Body In female mammalian cells, there is a dark spot of chromatin visible at the edge of the nucleus during interphase. This enables the sexing of cells. (used for verification of sex in the Olympics) This is called a Barr Body and is one of the X chromosomes which has become dense. Most of the genes on the Barr Body have become inactive.

The Barr Body In females only one X chromosome is fully active. Barr bodies are reactivated in the cells of the gonads that undergo meiosis to form gametes. Which of the 2 Xs forms the Barr body is random in different cells.

The Barr Body Thus females are a mosaic of 2 types of cells, those where the father’s X chromosome has become the Barr body and those where the mother’s X chromosome has become the Barr body. This can lead to differentiation between cells, as in variegation.

Variegation Some mice and cats have several sex-linked genes for certain coat colours. A female heterozygous for such genes may show patches of 1 coat colour in the midst of an area of another colour. This is known as variegation and is evident in calico and tortoiseshell cats.

Variegation 1 X chromosome becomes inactive (forms a Barr body) early in development, after that all the cells that arise from that particular cell have the same inactive X chromosome. In other cells, it is the other X that becomes deactivated.

Variegation This results in 1 colour coming from 1 group of cells and a different colour coming from another group of cells. In torotiseshell cats one X codes for orange and the other codes for black.

Sex-Linked Genes In mammals part of the X chromosome is homologous with the Y chromosome. Since the Y chromosome is so much smaller, there are parts of the X chromosome that have no matching part on the Y chromosome. Any gene carried on the non-homologous part of the X chromosome is called sex-linked.

Sex-Linked Genes The Y chromosome is genetically empty for that characteristic. Red-green colour blindness and haemophilia in humans. Red/white eye colour in fruit flies All tortoiseshell cats are female

Sex-Linked Genes For males, any faulty genes on the X chromosome will show up because the Y cannot mask the effect. In females both would have to carry a recessive trait before it would show up. In cases like red-green colourblindness where the allele is very common, some females are affected.

Sex-Linked Genes Refer to pg 116/117 wkbk or 167 txtbk Case study: Haemophilia.

Family Trees There are three common types of family tree: Sex-linked recessive Autosomal recessive Autosomal dominant An autosome is any chromosome that is not a sex chromosome.

Family Trees Refer to pg 114 - 115 wkbk Pg 169 - 171 txtbk Questions 1-4 Pg 118 – 121 wkbk self-check