LECTURE 18 VARIATION AND SEX LINKAGE VARIATION • Refers to difference in characteristics shown by organisms belonging to the same population or species. • There are two types of phenotypic variation and these are continuous and discontinuous variation. 1. Continuous Variation • This is a type of variation in which one character is controlled by many genes (polygenes) working together to produce a phenotype. • It involves characters that show a wide range of values from one extreme to the other within a population. • Such characters are produced by a combined effects of many gene and the environment. • The frequency distribution of a polygenic character is a normal distribution. • The majority of individuals in the population will have a phenotypic value in the middle with very few showing extreme values. • Continuous variation is also called quantitative variation. • Example of polygenic traits in human beings include skin colour, height, body weight etc. • In plants, yield is a polygenic trait controlled by many genes such as those responsible for germination, photosynthesis, amount roots and drought tolerance. 2. Discontinuous variation • This is a type of variation in which individuals show clear cut differences between different phenotypes. • Examples of discontinuous variation include gender, eye colour and blood type in human. • Characteristic showing discontinuous variation are usually controlled by one gene (monogenic) or by two more genes (oligogenic) with two more allelic forms. • Their phenotype is relatively unaffected by the environment. • This form of variation is also called quantitative variation. Who can roll their tongue like me!? Some examples of Discontinuous Variation. QUANTITATIVE TRAITS (POLYGENIC TRAITS) 1. Show continuous variation. QUALITATIVE TRAITS (OLIGOGENIC TRAITS) Show discontinuous variation. 2. Controlled by many genes. Controlled by one or more genes. 3. Grouping in distinct classes is not possible. Grouping in distinct classes is possible. 4. Metric measurement is possible. Metric measurement is not possible. 5. Analysed by complex statistics e.g variance, std. Analysed by simple statistics e.g frequencies. Variation and Selection • Variation enables some organisms to survive better (compete more successfully) • The ones with beneficial alleles survive, breed and pass on their alleles to the next generation • Those without beneficial alleles die before they reproduce, so their alleles are less likely to be passed on. • The beneficial alleles increase in frequency in the population and may eventually produce a new species. SEX DETERMINATION This is the establishment of the sex of organism through inheritance of sex chromosome. • In humans, sex is determined by the sex chromosome X and Y. • The Y chromosome determines maleness and X chromosome determine femaleness in the absence of Y. • A females therefore have 2 complimentary sex chromosomes XX in the eggs . • Males have 2 non-complementary sex chromosomes XY in their sperms. • The other 22 pairs of chromosomes are non sex chromosome also known as autosomal chromosome denoted by letters AA. • The Y chromosome is much smaller than the X and it carries a small number of genes, most of which are for “male characteristics”. • The X chromosome contains genes that code for all aspects of femaleness and also genes unrelated to gender. e.g genes for vision and immunity. • 50% of the male gametes carry an X chromosome and 50% carry the Y chromosome. • Therefore, males in humans are responsible for determining the sex of the offspring. • In mammals the homogametic sex is female (XX) and the heterogametic sex is male (XY) SEX LINKAGE • Genes carried on the sex chromosome are said to be sex linked. • The characters whose genes are located on sex or ‘X’ chromosomes are known as sex linked traits. • Such genes are called sex linked genes and linkage of such genes is referred to as sex linkage. • Inheritance of such genes or characters is known as sex linked inheritance. SEX LINKED TRAITS • Genetic disorders such as haemophilia and colour blindness are good examples of sex linked traits. • They are said to be sex linked traits because males (XY) develop these traits more than females (XX). • Y chromosome does not carry alleles homologous to those on the X chromosome. • Thus when it come to sex linked traits males can only carry one allele found on their Xchromosome. • This one allelic condition is termed as hemizygous. • It is this hemizygous condition which causes males to show expression even from a single recessive gene, a condition referred as pseudodominance. • Females on the other hand can be homozygous and heterozygous for sex linked trait because they carry two X chromosome. • A single recessive can not be expressed in females because they have a second X chromosome which may carry a dominant allele to counteract the recessive one. • Therefore, the sex linked traits are more likely to be visible in males since they can not have dominant allele to counteract the recessive on their X chromosome. • A study of genetic disorder such as haemophilia and colour blindness show how hereditary characters can show peculiar relationship to sex. Sex linkage in Drosophila • The first demonstration of sex linkage was the red eye colour gene in Drosophila, the fruit fly. • Normal fruit fly eye colour is a dull brick red. Mutations in this gene cause the eyes to be white. • The white allele is recessive, but it was quickly determined that the inheritance pattern for this gene was different from those of other genes being studied, which are located on chromosomes other than sex chromosomes (autosomes). • It turned out that this particular eye colour gene was located on the X chromosome. • The red eye phenotype is dominant over the white eye phenotype. • As females have two chromosomes X (with a locus for eye color), they might be homozygous or heterozygous for either allele. • Males, who carry only one X chromosome, are always hemizygous. • They carry only the one X chromosome inherited from their mother, and it determines their eye colour. Let us consider; Scenario 1 • The results of mating between homozygous red eyed females (++) mate with hemizygous white eyed males (-). • In the offspring, all the daughters shall be red eyed heterozygotes (+-) and all sons shall be red eyed hemizygotes (+-). Scenario 2 • In contrast, when the homozygous white eyed females (--) mate with hemizygous red eyed males (+). • In the offspring, all the daughters shall be red eyed heterozygotes (+-) and all sons shall be white eyed hemizygotes (-). • A male will show the X-linked recessive trait due to receiving only a single copy of the allele, because he has no second X chromosome to carry a dominant allele which might hide the recessive. • Females must inherit the recessive trait twice to show it, just as they do for any other recessive trait. • It is more likely for the male to show recessive trait in their phenotype because all they need one recessive allele on their X chromosome. • There are also a very few genes which are Ylinked (or holandric). • Y-linked genes are carried on the Y chromosome, and are thus passed directly from father to son. • Every son has a copy of his father’s Y chromosome END OF LECTURE! THANK YOU.
