Recombination and Mendel's 2nd Law Flashcards
Dihybrid cross peas
P1: Smooth Yellow x Wrinkled Green (SSYY x wwgg)
F1: All Smooth Yellow (SwYg)
F2: 9:3:3:1 (SY:Sg:wY:wg)
Mendel’s 2nd Law
(Law of independent assortment)
During formation of gametes, segregation of alleles at one locus is independent of that at another locus
Number of heterozygous loci
n
No. possible types sperm and egg
2^n
No. F2 genotypes
3^n
No. F2 phenotypes
2^n
Incomplete dominance
Expression of heterozygote is an intermediate between the dominant heterozygotes
Example and F2 ratio of incomplete dominance
Cross red x white → pink
F2 ratio = 1R:2P:1W
Codominance
Phenotype of gene’s allele are fully and simultaneously expressed in heterozygote
Examples of codominance
Cross red cows x white cows → red cows with white spots
Sickle cell haemoglobin
Multiple Alleles
More than 2 alleles at a locus
Example of multiple alleles
Drosophila: wild type (+/+) white eye (w/w)
Several alleles involved in making ‘apricot’ eyes
Human ABO blood group (also codominance)
Lethal alleles
Homozygotes can be lethal
Examples of lethal alleles
Cross yellow mouse x wild type P1: Yy x yy F1: 2Yy:2yy suggests yellow dominance Cross yellow x yellow P1: Yy x Yy F1: 1YY (Dies) : 2Yy : 1yy Appears as 2 yellow : 1 wildtype Also Achondroplastic dwarfism in humans
Pleiotropy
One gene affects many characters
Examples of pleiotropy
All blue-eyed white cats are deaf, both due to melanin
Tabby cats are aggressive
Change in single base → sickle cell has many other detrimental effects
Gene interaction
Several genes affect one character - alleles at one locus alter phenotype produced by an allele at a different locus
Examples of gene interaction
Sex limitation in males
Mouse coat colours = 5 interacting loci
Autism
Secretor (Se) locus, interacts with ABO locus, SeSe or Sese produce ABO blood group substances in saliva + other bodily fluids
Mouse coat colour (A locus)
Determines distribution of colour in hair shaft
Wild type = agouti, dark shaft + yellow band, dominant, A
Mouse coat colour (B locus)
Codes for colour of hair
Black, dominant, B
Brown, recessive, b
If they’re agouti and brown = cinnamon in F2, brown with yellow fleck
Mouse coat colour (C locus)
Presence of absence of pigment Pigment, dominant, C Albino, recessive, c Albino is epistatic, prevents other genes expressing their effects P1: BBCC(black) x bbcc(albino) F1: BbCc (all black) F2: 9 B-C-(black): 3 bbC-(brown): 3 B-cc(albino): 1 bbcc(albino) Phenotypic ratio 9:3:4
Mouse coat colour (D locus)
Expression of pigment Full expression, dominant, D Diluted, recessive, d P1: BBDD(black) x bbdd (dilute brown) F1: BbDd (all black) F2: 9 B-D-(black): 3 bbD-(brown): 3 B-dd(dilute black): 1 bbdd(dilute brown)
Mouse coat colour (S locus)
Controls pigment distribution on body
Plain, dominant, S
Piebald, recessive, s
Complementation
2 strains of an organism with different homozygous recessive mutations that produce same phenotype
Complementation example
Cross white (recessive) foxgloves from different populations → purple (dominant) F1
P1: w1w1W2W2 x W1W1w2w2
F1: W1w1W2w2 (all purple)
F2: 9 W1-W2-(purple): 3 W1-w2w2(white): 3 w1w1W2-(white): 1 w1w1w2w2 (white)
Phenotypic ratio = 9:7
In humans: many loci involved in hearing, complementation can give normal child from deaf parents
Complementation test
Cross drosophila blue eyed recessive x green eyed recessive - both homozygotes
If they are at same locus, F1 = all blue or all green (depending on dominance)
If 2 separate loci involved F1 = all red as they will be heterozygous and red is dominant
Links between environment and deafness
Wardenburg syndrome - syndromic deafness linked to melanin
Usher syndrome - deaf + blind - pleiotropy
Women with ‘Derbyshire neck’ from iodine shortage can have deaf children
Pregnant women with German measles can have deaf children
Human complementation - Thalassemia + Sickle Cell
Thalassemia - inherited autosomal recessive blood disorder → less haemoglobin in RBC
Can be good for people who also have sickle cell disease as there is less haemoglobin packed into the smaller cells