B11.3 : Inheritance

Question 1

Alleles

Answer 1

1. Alleles are different forms of the same gene. They are a version of a gene
2. Humans have pairs of every gene and in one gene, each half of the pair may have different alleles.
3. People’s characteristics are determined by the alleles that they have.
4. Alleles can either be dominant or recessive

Question 2

Dominant Alleles

Answer 2

1. A dominant allele is always expressed, regardless of the identity of the other allele.
2. It only needs one copy present to be expressed (BB or Bb).
3. It is represented by a capital letter, e.g. B.
4. If B is the allele for brown eyes: When a person has a copy of the B allele, they will have brown eyes, no matter what other allele is present.

Question 3

Recessive Alleles

Answer 3

1. A recessive allele is only expressed if the other allele is also recessive.
2. It is represented by a lowercase letter e.g. b. It needs two copies to be present to be expressed (bb).
3. If b is the allele for blue eyes: A person can only have blue eyes if both of their alleles are b.

Question 4

Characteristics and genes

Answer 4

1. In most cases, a characteristic results from multiple genes interacting.
2. However, sometimes, a single gene is responsible for a characteristic.
3. Red/green colour blindness is an example of a characteristic determined by a single gene.

Question 5

Genotype

Answer 5

1. Genotype refers to the combination of alleles an organism has.
2. If the two alleles are the same, we say that the person is homozygous (BB or bb).
3. Breeding two identical homozygous individuals is called pure-breeding.
4. If the two alleles are different, we say that the person is heterozygous (Bb).
5. Two heterozygous individuals breeding is not pure-breeding

Question 6

homozygous

Answer 6

1. having two identical alleles of a particular gene
2. if both are dominant - homozygous dominant
3. if both are are recessive - homozygous recessive

Question 7

heterozygous

Answer 7

1. having two different alleles of a particular gene

Question 8

Phenotype

Answer 8

1. A phenotype is an observed characteristic of an individual.
2. The phenotype is determined by the interaction between the genotype and environment.
3. Earlobes being attached or free is an example of a phenotype, where the alleles present will determine a characteristic, unless the environment interferes.

Question 9

Punnett Squares

Answer 9

1. Punnett squares are diagrams that help us to visualise the outcome of a monohybrid cross. In these diagrams, a capital letter shows a dominant allele.
2. The alleles of the parents are drawn along the top and side of a grid.
3. The pairs of alleles that the offspring could have are then filled in to the grid.

Question 10

Pea plant punnett squares

Answer 10

1. Pea plants can be either green or yellow. The green colour allele is dominant over the yellow allele.
2. We represent the dominant green allele with G, and the recessive yellow allele with g on a Punnett square.
3. With one homozygote recessive parent and one heterozygote parent (shown above):
4. There is a 50% chance of offspring being yellow.
5. Ratio of green:yellow offspring will be 1:1.

Question 11

Monohybrid inheritance

Answer 11

1. Monohybrid inheritance refers to the inheritance of traits determined by a single gene.

Question 12

Cystic Fibrosis Pedigree Diagram

Answer 12

1. We can use the knowledge that cystic fibrosis is caused by a recessive allele to work out the allele combinations of each individual in a Pedigree diagram.
2. If one of their offspring is a sufferer, and neither of the parents is a sufferer:
3. The offspring must have two recessive alleles in order to suffer.
4. The parents are non-sufferers, so must have at least one of the dominant, non-recessive alleles each.
5. If either parent were homozygote dominant, the child could not be a sufferer.
6. Therefore, the parents must both be heterozygotes.

Question 13

Pedigree Diagrams

Answer 13

Pedigree diagrams can be used to visualise the transmission of inherited disorders from one generation to the next.

Question 14

Alleles in gametes

Answer 14

1. Each gamete has only one of each kind of chromosome instead of two, as in the body cells
2. e.g. human egg and sperm cells has 23 chromosomes, not 46 as in other cells
3. These cells therefore only carry one pair of alleles of all the genes

Question 15

Sex determination

Answer 15

1. The sex chromosomes are responsible for determining what sex a person will be
2. A woman’s chromosomes are both alike and are called X chromosomes. She has the genotype XX
3. A man though only has one chromosome. the other one (which is smaller) is a Y chromosome. He has the genotype XY