Monogenic inheritance

Question 1

Monogenic inheritance

Answer 1

When a phenotype or trait is controlled by a single gene

Question 2

Steps in analysing a genetic cross:

Answer 2

• State phenotype of both parents
• State genotype of both parents, assigning a letter code to represent the alleles of the gene being studied.
• State the gametes of each parent. It is common practice to circle the letters.
• Use a Punnett Square to show the results of the random fusion of gametes during fertilisation. Label the gametes on edges of square
• State the proportion of each genotype which are produced among the offspring. This can be in the form of a percentage or ratio.
• State the corresponding phenotype for each possible genotypes.

Question 3

Mendel’s experiments

Answer 3

• Pea plants
• Two coloured pea pods (green and yellow)
• Allele for green pod plants are dominant.

Question 4

Homozygous genetic cross

Answer 4

• Deals with true breeding or pure breeding
• All the offspring will be heterozygous.

Question 5

F1 and F2

Answer 5

• F1 is first generation produced using punnet squares
• F2 is second generation produced using punnet squares

Question 6

Codominance

Answer 6

When two different alleles occur for a gene - both of which are equally dominant.
- Both genes are expressed in the phenotype of the organism present.

Question 7

Codominance in snapdragon flowers:

Answer 7

• White flowers - production of pigment not catalysed by allele
• Red flowers - production of red pigment catalysed by allele

Question 8

Results of codominance in snapdragon flowers:

Answer 8

• Red flowers - homozygous for the allele coding for red pigment
• White flowers - homozygous for the allele coding for white pigment
• Pink flowers - heterozygous.

Question 9

Representing codominance

Answer 9

Both capital letters used.
One has C^W and the other is C^R

Question 10

Multiple alleles

Answer 10

When a gene has more than 2 variations

Question 11

Immunoglobulin gene (Gene 1)

Answer 11

• Codes for different antigens present on surface of red blood cells

Question 12

Different alleles in immunoglobulin gene:

Answer 12

I^A - results in production of antigen A
I^B - results in production of antigen B
I^O - results in production of neither antigen

Question 13

Four blood groups

Answer 13

A - I^A, I^A or I^A, I^O
B - I^B, I^B or I^B, I^O
AB - I^A, I^B
O - I^O, I^O

Question 14

Determining sex

Answer 14

• The 23rd gene is sex chromosome
• Females have XX chromosomes and males have XY chromosomes.
• X chromosome is large and contains many genes not involved in sexual development.
• Y is very small, containing almost no genetic information, but does carry a gene that causes embryo to develop into male.
• Sex is determined by whether a sperm cell contains an X or Y chromosome

Question 15

Sex linked

Answer 15

• Characteristics determined by genes carried on the sex chromosome

Question 16

Explanation of sex linkage

Answer 16

• Y chromosome is smaller than X chromosomes.
• Many genes males only have one copy of.
• Any recessive allele on the section of X chromosome that is missing on Y occurs in males more frequently.

Question 17

Haemophilia

Answer 17

• Sex-linkage genetic disorder
• Blood clots extremely slowly due to lack of protein blood-clotting factor (often factor VIII).
• Prolonged and untreated bleeding can be fatal

Question 18

Who are carriers of haemophilia?

Answer 18

• Women who have heterozygous genes coding for haemophilia

Question 19

X^H - Haemophilia

Answer 19

Represents the healthy allele

Question 20

X^h - Haemophilia

Answer 20

• Represent the recessive allele coding for haemophilia (through the non-production of blood-clotting protein). Known as a faulty allele

Question 21

Y - Haemophilia

Answer 21

Represents Y chromosome which has no allele attached

Question 22

Can a male with haemophilia pass on disorder to his daughters?

Answer 22

Yes