- Genetic Lineage and Pedigree Diagrams (2) -
Cystic fibrosis is an inherited disorder caused by a recessive gene. This disorder affects mucus production causing the blockage of tiny air passages in the lungs. Study the pedigree diagrams below and answer the questions that follow.
Name the genotypes represented by 1 and 3 in the diagrams respectively. (2)
What is Winnie’s genotype? (1)
Does Winnie suffer from cystic fibrosis? (1)
Winnie and Langa intend to get married. Draw a pedigree diagram to illustrate ALL the possible genotypes of their sons. (4)
Answer
Explain what Winnie and Langa should do before deciding to have children. (3)
Understand that there will be a 50% chance of having a normal child and 50% chance of having a child with cystic fibrosis.
Go for genetic counseling.
Accept responsibility for their decision/make an informed decision.
In humans the gene responsible for the clotting of blood is carried on the X chromosome. Males who carry the recessive gene, bleed easily and are called haemophiliacs. Females will only have haemophilia if they have two recessive genes. The diagram below shows the occurrence of haemophiliacs in a certain family after surveys were carried out.
Let H = normal clotting (XH) and h = haemophilia (Xh)
The genotype of individual 5 is indicated in the following diagram.
Write down the genotypes of the individuals 1 to 4. (8)
Study the diagrams below that show some breeding experiments on mice. A single pair of alleles showing complete dominance controls coat colour (white or grey) in these mice.
State which sex chromosomes would be present in the gametes of parent 2 and mouse 3, respectively. (2)
Mouse 2 - XY
Mouse 3 - XX
If mice 3 and 4 had a second set of offspring, what is the probability that the first mouse born would be a female? (1)
Which of the parent mice (1, 2, 3 or 4) is likely to be homozygous dominant for coat colour? (1)
State why mouse 3 can only be heterozygous for coat colour. (2)
A cross between mouse 3 and mouse 4 produced offspring with white /recessive coat colour and white colour will only show up if both parents have at least one recessive gene.
Study the family tree below which shows inheritance of the type of little finger over four generations of a family.
How many members of the family have straight little fingers? (1)
Is person F homozygous or heterozygous for the type of little finger? (1)
Which type of little finger is controlled by a dominant gene? Explain your answer. (3)
Bent little finger.
Only two parents showing dominant features can produce offspring showing both phenotypes/bent and straight little finger. OR
Two parents with straight little fingers must be homozygous recessive to produce offspring all with only straight little fingers.
Because parents A and B had a son with a straight little finger.
In the fourth generation of the family, what proportion would be female with a straight little finger? (2)
Persons F and G are twins. Were they produced from a single fertilised egg cell? Give TWO reasons based on the phenotype to support your answer. (3)
No.
G is male and F is female/different sexes/non-identical/fraternal.
F has a bent little finger and G has a straight little finger.
Study the following diagram which shows three generations of snapdragon plants and answer the questions which follow.
Use the following symbols for the contrasting alleles:
W – for white flowers R – for red flowers
State the kind of dominance shown in the diagram above. (1)
Use the symbols R and W and write down the genotypes of each of the following snapdragon plants: A ; B ; C respectively. (6)
The diagram below shows a cross between a homozygous black mouse and a homozygous white mouse. The F1-generation was all black.
Use a genetic cross to show the possible genotypes and phenotypes of the F2 generation, if mouse 1 and 3 breed. Use the symbols B and b for the alleles of fur colour. (7)
Answer
Questions source: DBE (Authorisation 11798)
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