Recombination and Mendel's 2nd Law

Question 1

Dihybrid cross peas

Answer 1

P1: Smooth Yellow x Wrinkled Green (SSYY x wwgg)
F1: All Smooth Yellow (SwYg)
F2: 9:3:3:1 (SY:Sg:wY:wg)

Question 2

Mendel’s 2nd Law

Answer 2

(Law of independent assortment)

During formation of gametes, segregation of alleles at one locus is independent of that at another locus

Question 3

Number of heterozygous loci

Answer 3

n

Question 4

No. possible types sperm and egg

Answer 4

2^n

Question 5

No. F2 genotypes

Answer 5

3^n

Question 6

No. F2 phenotypes

Answer 6

2^n

Question 7

Incomplete dominance

Answer 7

Expression of heterozygote is an intermediate between the dominant heterozygotes

Question 8

Example and F2 ratio of incomplete dominance

Answer 8

Cross red x white → pink

F2 ratio = 1R:2P:1W

Question 9

Codominance

Answer 9

Phenotype of gene’s allele are fully and simultaneously expressed in heterozygote

Question 10

Examples of codominance

Answer 10

Cross red cows x white cows → red cows with white spots

Sickle cell haemoglobin

Question 11

Multiple Alleles

Answer 11

More than 2 alleles at a locus

Question 12

Example of multiple alleles

Answer 12

Drosophila: wild type (+/+) white eye (w/w)
Several alleles involved in making ‘apricot’ eyes
Human ABO blood group (also codominance)

Question 13

Lethal alleles

Answer 13

Homozygotes can be lethal

Question 14

Examples of lethal alleles

Answer 14

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

Question 15

Pleiotropy

Answer 15

One gene affects many characters

Question 16

Examples of pleiotropy

Answer 16

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

Question 17

Gene interaction

Answer 17

Several genes affect one character - alleles at one locus alter phenotype produced by an allele at a different locus

Question 18

Examples of gene interaction

Answer 18

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

Question 19

Mouse coat colour (A locus)

Answer 19

Determines distribution of colour in hair shaft

Wild type = agouti, dark shaft + yellow band, dominant, A

Question 20

Mouse coat colour (B locus)

Answer 20

Codes for colour of hair
Black, dominant, B
Brown, recessive, b

If they’re agouti and brown = cinnamon in F2, brown with yellow fleck

Question 21

Mouse coat colour (C locus)

Answer 21

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

Question 22

Mouse coat colour (D locus)

Answer 22

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)

Question 23

Mouse coat colour (S locus)

Answer 23

Controls pigment distribution on body
Plain, dominant, S
Piebald, recessive, s

Question 24

Complementation

Answer 24

2 strains of an organism with different homozygous recessive mutations that produce same phenotype

Question 25

Complementation example

Answer 25

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

Question 26

Complementation test

Answer 26

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

Question 27

Links between environment and deafness

Answer 27

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

Question 28

Human complementation - Thalassemia + Sickle Cell

Answer 28

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