Linkages And Epistasis Flashcards
Explain what is meant when some characteristics are sex-linked?
-the genetic information for biological sex is carried on 2 sex chromosomes
-in mammals, females have 2 X chromosomes (XX) and males have 1 X and 1 Y chromosomes (XY)
-a characteristic is said to be sex-linked when the allele that codes for it is located on the sex chromosomes
-the Y chromosome is smaller than the X and carries fewer genes, so most genes on the sex chromosomes are only carried on the X chromosomes
-As males only have 1 X chromosome, they often only have 1 allele for sex-linked genes, so because they only have 1 copy they express this characteristic even if it’s recessive. This makes males more likely then females to show recessive phenotypes for genes that are sex-linked
-genetic disorders caused by faulty alleles on the sex chromosomes include red-green colour blindness and haemophilia, and are X-linked disorders
Explain this genetic diagram showing the sex-linked disorder colour blindness
-because it is sex-linked, you must show both the chromosomes (X & Y) and the alleles (N =normal colour vision & n = faulty colour vision)
-the Y chromosome however doesn’t have an allele for colour vision so is just represented by Y
-females would need 2 copies of the recessive allele to be colour blind (Xn Xn) but males only need one (Xn Y)
-this shows colour blindness is much rarer in women then in men
-there is a 3:1 ratio of offspring without colour blindness : offspring with colour-blindness
-also a 2:1:1 ratio of female offspring without colour blindness : male offspring without colour blindness : male offspring with colour blindness
-this ratio would change if the carrier female crossed with a colour blind male to 1:1
Explain how some autosomal genes are linked (chromosomes which aren’t sex-linked)
-because genes are on the same autosome, they’ll stay together during the independent assortment of chromosomes in meiosis 1, and their alleles will be passed on to the offspring together. This won’t happen if crossing over splits them up first.
-the closer the 2 genes are on the autosome, the more closely they are said to be linked, because crossing over is less likely to split them up
-if 2 genes are autosomally linked, you won’t get the phenotypic ratio you expect in the offspring cross. Dihybrid crosses usually get a phenotypic ratio of 9:3:3:1, but it is more likely to be a 3:1 ratio because the 2 autosomally-linked alleles are inherited together
-we would normally write BbVv as the genotype, but if the genes are linked we write BV bv showing the alleles are inherited together
What is epistasis?
-the interaction between 2 non-linked genes which causes one gene to mask the expression of the other gene in the phenotype
-there are 3 different types of epistasis: recessive, dominant and complementary
What is the difference between antagonistic and complementary epistasis
-antagonistic works against each other, 1 leads to masking of the other
-complementary work with each other, 1 encourages the expression of the other
What is the epistatic gene?
The gene that does the suppressing when a gene suppresses another gene
What is the hypostatic gene?
The gene which is being suppressed
What is recessive antagonistic epistasis? Give an example
-occurs when the presence of two copies of the recessive allele at the first locus prevents the expression of another allele at a second locus
-E.g the control of flower colour in the Salvia plant. In the hypostatic gene, a dominant B causes purple flowers, but recessive bb causes pink. The flower can only be coloured however if the epistatic gene has a dominant A allele. If there are 2 recessive aa, the hypostatic gene is blocked and the flower is white.
A purple flower = AaBB or AaBb
A pink flower = Aabb
A white flower = aaBb or aaBB or aabb
What is dominant antagonistic epistasis? Give an example
-occurs when the expression of the dominant allele of the epistatic gene prevents the expression of the hypostatic gene
-E.g in the inheritance of feather colours in chickens. In the hypostatic gene a dominant C allele causes coloured fur, whilst recessive cc will give white fur. In the epistatic gene, if a dominant I allele is present (II or Ii) this antagonises the hypostatic gene and will give white fur regardless of the alleles. Coloured fur can only be expressed if the epistatic gene is double recessive (ii) and then the hypostatic gene has a dominant C (CC or Cc)
What is complementary epistasis? Give an example
-occurs when the two genes work together with each other
-E.g inheritance of coat colour in mice. The presence of a dominant allele C in the epistatic gene produces coloured fur (CC or Cc). The recessive cc genotype stops the formation of pigment, giving a WHITE mouse. The hypostatic gene determines the distribution of the pigment if it is made. The dominant A allele gives agouti hair (AA or Aa) and double recessive aa gives just black hair.
Agouti fur = CcAA or CCAA or CcAa
Black fur = Ccaa or CCaa
White fur = ccAA or ccaa or ccAa
