Chapter 4 -Mendelian Genetics

Question 1

Define Homozygous

Answer 1

Same genes eg AA

Question 2

Define Heterzygous

Answer 2

Different genes eg Aa

Question 3

Define Genotype

Answer 3

Genetic makeup of the organism

Question 4

Define Phenotype

Answer 4

Physical makeup of the organism

Based on the genotype and environment

Question 5

Answer a punnet square question

Answer 5

1. Define alleles
2. Do cross
3. Use punnet square
4. % genotype and % phenotype

Question 6

Monohybrid cross

Answer 6

A cross showing the probabilities of genotypes and phenotypes from one pair of alleles that determine one trait

Question 7

Test cross

Answer 7

Parents showing dominant trait could be TT or Tt

• A test cross is the matting/crossing of an organism showing the dominant phenotype, to determine its genotype
• Always across with a homozygous recessive organism. If some or any offspring show the recessive trait, the organism in question must be heterozygous.
• To identify whether an organism exhibiting a dominant allele is homozygous or heterozygous for a specific allele, a scientist can perform a test cross.
• The organism in question is crossed with an organism that is homozygous recessive for that same trait, and the offspring of the test cross are examined - If the offspring produce any recessive traits, it indicates the organism in question is heterozygous.

Question 8

Incomplete dominance

Answer 8

One trait isn’t dominant over the other so neither is expressed
Eg RR x WW = Pink
R = red. W = white

Question 9

Codominance

Answer 9

The alleles for a trait are equally expressed

Eg WW x RR = roan

Question 10

Multiple allele system

Answer 10

Multiple alleles exist in a population when there are many variations of a gene present. In organisms with two copies of every gene (diploid organisms) each organism has the ability to express two alleles at the same time.
- In mono hybrid crosses we have one pair (2) genes that determine a trait.
- In multiallelic systems, there are more than 2 alleles.
- In the case of the ABO blood group system, there are 3 potential alleles I ᴬ, I ᴮ, i.
▪ Iᴬ = immunoglobulin A
▪ Iᴮ = immunoglobulin B
▪ i = no markers
- Iᴬ or Iᴮ is dominant to i.
- Iᴬ is codominant to Iᴮ

Question 11

Possible genotypes for blood group A

Answer 11

IᴬIᴬ = A
Iᴬi = A

Question 12

Possible genotypes for blood group B

Answer 12

IᴮIᴮ = B
Iᴮi = B

Question 13

Possible genotypes for blood group O

Answer 13

ii = O

Question 14

Possible genotypes for blood group AB

Answer 14

IᴬIᴮ = AB

Question 15

Sex linked genetics

Answer 15

Sex linked genetics or X-linked, refers to the inheritance pattern observed when studying genes carries on the X-chromosome.
Males only inherit one X-chromosome. Therefore, they are said to be hemi-zygous for genes on the X-chromosome.

Question 16

Sex linked genetics rules

Answer 16

1. If the female is affected (X ᴴ X ͪ), then all daughters are all at least carriers. All sons are affected.
2. If the male is affected (X ͪ Y,) then all the daughters are at least carriers.
3. If the mother is a carrier (X ᴴ X ͪ) 50% of the sons will be affected. 50% of the daughters will at least be a carrier.

Question 17

Dominant sex linked genetics

Answer 17

R= retts syndrome
r = normal
 X ᴿ X ᴿ = retts syndrome
X ᴿ X ͬ = retts syndrome
X  ͬ X  ͬ = normal
X ͬ Y = normal
X ᴿ Y = retts syndrome

Question 18

Dihybrid cross

Answer 18

A punnet square where you can predict the probabilities of 2 contrasting characteristics at the same time, using 2 pairs of genes

Remember 9:3:3:1
( dominance dominance : dominance recessive : recessive dominance : recessive recessive)

Question 19

Sex linked condition examples

Answer 19

Red and green colour blindness and haemophilia and DMD (Duchene Muscular Dystrophy).

Question 20

Define allele

Answer 20

Remember alternate forms of a gene

Eg different coloured eyes