6. Inheritance, Variation and Evolution (alleles, punnet squares genetic inheritance)

Question 2

What are alleles?

Answer 2

different versions of the same gene

Question 3

What is a dominant allele?

Answer 3

A version of a gene where only one copy is needed for it to be expressed

Question 4

What is a recessive allele?

Answer 4

A version of a gene where two copies are needed for it to be expressed

Question 5

What is meant when an organism is homozygous?

Answer 5

When an organism has two copies of the same allele (two recessive or two dominant)

Question 6

What is meant when an organism is heterozygous?

Answer 6

When an organism has two different versions of the same gene (one dominant and one recessive)

Question 7

What is the genotype?

Answer 7

the genes present for a trait

Question 8

What is the phenotype?

Answer 8

the visible characteristic

Question 9

How are dominant alleles represented in a punnet square?

Answer 9

Using uppercase letters

Question 10

How are recessive alleles represented in a punnet square?

Answer 10

a lowercase version of the same letter as the dominant allele

Question 11

What is the problem with single gene crosses?

Answer 11

most characteristics are controlled by multiple alleles rather than just one

Question 12

What is an inherited disorder?

Answer 12

A disorder caused by the inheritance of certain alleles

Question 13

Give 2 examples of inherited disorders?

Answer 13

• Polydactyly (having extra fingers or toes) is called by a dominant allele
• Cystic fibrosis (a disorder of cell membranes) caused by a recessive allele

Question 14

How are embryos screened for inherited disorders?

Answer 14

• During IVF, one cell is removed (from an 8 cell embryo) and tested for disorder causing alleles
• If the cell doesn’t have any indicator alleles, then the originating embryo is implanted into the uterus

Question 15

What are the ethical issues concerning embryo screening?

Answer 15

• It could lead to beliefs in society that being disabled or having a disorder is less human or associated with inferiority
• The destruction of embryos with inherited disorders is seen by some as murder as these would go on to become human beings
• It could be viewed as part of the concept of designer babies as it may be for the parents convenience or wishes rather than the child’s wellbeing

Question 16

What are the economic issues concerning embryo screening?

Answer 16

costs of hospital treatment and medication will need to be considered if it is known that a child will have an inherited disorder and financial support explored if necessary

Question 17

What are the social issues concerning embryo screening?

Answer 17

• social care for children with inherited disorders may need to be considered if parents are unable to provide care
• if an embryo is found to have an inherited disorder and is terminated, this can prevent a child and its parents from potential suffering in the future due to the disorder

Question 18

What is gene therapy?

Answer 18

The insertion of a normal allele into the cells of a person with an inherited disorder to functionally replace the faulty allele

Question 19

What are the ethical issues concerning gene therapy?

Answer 19

• Some people believe that it is going against and ‘playing God’
• The introduced genes could enter sex cells and so be passed to future generations

Question 20

What are sex chromosomes?

Answer 20

A pair of chromosomes that determine sex:
- males have an X and a Y chromosome
- females have two X chromosomes

Question 21

Why does the inheritance of a Y chromosome mean that an embryo develops into a male?

Answer 21

Testes development in an embryo is stimulated by a gene present on the Y chromosome

Question 22

What is a sex-linked characteristic?

Answer 22

A characteristic that is coded for by an allele found on a sex chromosome

Question 23

Why are the majority of genes found on the X chromosome rather than the Y chromosome?

Answer 23

The X chromosome is bigger than the Y chromosome so more genes are carried on it

Question 24

Why are men more likely to show the phenotype for a recessive sex-linked trait than women?

Answer 24

• many genes are found on the X chromosome that have no counterpart on the Y chromosome
• Women (XX) have two alleles for each sex-linked gene, whereas men (XY) often only have one allele therefore only one recessive allele is required to produce the recessive phenotype in males