linkage Flashcards
in humans, just __ _____ of chromosomes carry the genes that determine many thousands of different characteristics. it follows that each chromosome must possess ____ _________ _____.
- 23 pairs
- many different genes
what does it mean if genes are linked?
any two genes that occur on the same chromosome are said to be linked
what are the 22 chromosomes that aren’t the sex chromosomes called?
the remaining 22 chromosomes, other than the sex chromosomes, are called autosomes
what are the two types of linkage and how are they defined?
sex linked - genes carried on the sex chromosome are said to be sex linked
autosomal linkage - the name given to the situation where two or more genes are carried on the same autosome is called autosomal linkage
what are sex linked genes and characteristics?
many genes carried on sex chromosomes are unrelated to sexual characteristics but are inherited in different ways in males and females. these are said to be sex linked genes and characteristics
in males, the sex chromosomes are ___-__________
what does this mean?
non homologous
any genes on the non homologous section of the X chromosomes will not have an equivalent allele on the Y chromosome. this means that a recessive allele on an X chromosome would have much more chance of being expressed in the phenotype of a male
when answering questions on sex linked characteristics, what do you need to take into account and identify?
when answering questions on sex linked characteristics, you need to take into account, and identify, both the sex and the phenotype for the condition
what symbols do you use to represent the sex linked alleles?
X^B, X^b, Y^-
where B/b are alleles for that gene. A - indicates no allele present.
what are examples of sex linked diseases caused by recessive alleles on X chromosomes?
examples of sex linked diseases caused by recessive alleles on X chromosomes are haemophilia, colour blindness and muscular dystrophy. such diseases are passed on from a carrier, heterozygous mother to her son, in most cases.
males always inherit their X from their ______ and pass the X on to _________.
mother, daughters
do genes in autosomal linkage segregate in accordance with Mendel’s Law of Independent Assortment?
assuming there is no crossing over, all the linked genes remain together during meiosis, and so pass into gametes, and hence the offspring together. they do not segregate in accordance with Mendel’s Law of Independent Assortment.
if the two genes controlling two different characters were on different homologous pairs of chromosomes… (diagram)
- if genes A and B occur on separate chromosomes, that is, they are not linked
- if genes A and B occur on the same chromosome, that is, they are linked
- two homologous pairs are needed if all four alleles are to be present
- according to Mendel’s Law of Independent Assortment, any one of a pair of characters may combine with any of another pair. there are therefore four different possible types of gamete.
if the two genes controlling two different characters were on the same homologous pair of chromosomes ie linked…
- if genes A and B occur on the same chromosomes, that is, they are linked
- only one homologous pair is needed if all four alleles are to be present
the inheritance of two characters, determined by two different genes located on different chromosomes can be represented using a ________ _____
dihybrid cross
how does the combinations of alleles in the gametes differ with non-linked and linked genes?
where the two genes A and B with heterozygous alleles are on different chromosomes, there are four possibly combinations of the alleles in the gametes: AB, Ab, aB and ab
however, if the two genes are linked and provided there is no crossing over, there are only two possible combinations of the alleles in the gametes: AB and ab
