Transcript SEX LINKAGE
SEX LINKAGE Characters which are associate more with one gender Sex Determination What are the odds of getting a male? A female? X Y X XX XY X XX XY 50/50 What is it? Sex Linkage is the phenotypic expression of an allele that is related to the chromosomal sex of an individual Since there are far more genes on the X chromosome than the Y, there are far more X linked traits that Y linked traits What’s the Comparison Like? Mode of Inheritance This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes What does autosomal mean? An autosome is a non-sex chromosome It is an ordinarily paired chromosome that is the same in both sexes of a species Mode of Inheritance Non-autosomal chromosomes are usually referred to as sex chromosomes, allosomes or heterosomes In contrast to normal autosomal chromosomes, both sexes do not have the same probability of expressing the trait Why does this happen? In mammals the female is said to be homogametic where as the male is said to be heterogametic. What does homogametic mean? It means that there are two of the same chromosomes (ie., X X) What does heterogametic mean? It means that there are two different chromosomes (ie., X Y) Why does this happen? The genes that are present on the X or Y chromosomes are called sex linked genes Now, the chromosomes of interest still have genes and alleles, and these alleles are passed on to their progeny Why does this happen? These traits are also, as we discussed, dominant and recessive It can be said that X-linked recessive traits are expressed in all heterogametics, and in homogametics that are homozygous for the recessive allele Well what does this mean? Heterogametics only have one X chromosome, and therefore what ever is there is expressed (ie., it can not be masked by the other chromosome) Homogametics on the other hand must have both chromosomes affected for the X linked recessive chromosome to have an effect Homogametic Heterogametic So what do you think? Which of the two, homogametic or heterogametic, do you think will have expression of the genetic disorders more often? Think to yourself, share with a friend, and then lets have a class discussion and make sure that your answer makes sense Males get the short end of the Stick Males, or heterogametics have a higher incidence of expression because of the fact that there is no other allele to mask the damaged one If this is the case, all males that get the gene or allele that codes for the damaged expression will express to the fullest extent So that’s the male, what about the female? What are the possibilities for the female? Is it possible for the female to express the same way that the male does? Yes, of course, but there is a stipulation The female must contain two damaged alleles (chromosomes) in order to express But hold on, what does this mean for the female with regards to passing on her genes? What can she be? She can actually be a carrier What is a carrier? A carrier is an individual that has two alleles, one damaged one and one regular Their phenotypes are the same as a regular individual But their genes hold a “secret” Back to men Can a man be a carrier? Think about it for a few minutes with a partner, and really think hard about it For X linked alleles, no it is not possible Why? What kind of progeny would we expect? The progeny We saw in the last photo that you can get normal males and females if the mother is a carrier, and you can also get carrier females and affected sons Why can you not get affected females? There is no affected X that comes from the father The only way to get an affected female is if there is an affected male and a carrier or affected female An Example Red/Green Colour blindness This is a recessive X linked When passed on to males, they are automatically colour blind to red and green, females can be carriers or can be colour blind Talking about it is easy How do you think we are going to represent the flow of genes through a family The best way is to draw a picture This picture that we are going to draw has a special name, its called a Pedigree What is a pedigree chart? Pedigree charts show a record of the family of an individual They can be used to study the transmission of a hereditary condition They are particularly useful when there are large families and a good family record over several generations. Studying human genetics You cannot make humans of different types breed together Pedigree charts offer an ethical way of studying human genetics Today genetic engineering has new tools to offer doctors studying genetic diseases A genetic counsellor will still use pedigree charts to help determine the distribution of a disease in an affected family Symbols used in pedigree charts Normal male Affected male Normal female Affected female Marriage A marriage with five children, two daughters and three sons. The eldest son is affected by the condition. Eldest child Youngest child Organising the pedigree chart A pedigree chart of a family showing 20 individuals Organising the pedigree chart Generations are identified by Roman numerals I II III IV Organising the pedigree chart Individuals in each generation are identified by Arabic numerals numbered from the left Therefore the affected individuals are II3, IV2 and IV3 I II III IV So now that we have seen that, lets try some problems