6.Inheritence , Variation and Evolution

Question 1

What is sexual reproduction ?

Answer 1

• Sexual reproduction involves the joining (fusion) of male and female gametes
• sperm and egg cells in animals
  • pollen and egg cells in flowering plants.
  In sexual reproduction there is mixing of genetic information which leads to variety in the offspring. The formation of gametes involves meiosis.

Question 2

What are gametes ?

Answer 2

A gamete is a sex cell produced in the sex organs by meiosis
They are:
• Sex cells (sperm cells and egg cells in animals, pollen and egg cells in flowering plants).
• Haploid (half the number of chromosomes).

Question 3

What is meiosis ?

Answer 3

• Form of cell division involved in the formation of gametes (non-identical haploid cells) in reproductive organs.
• Chromosome number is halved.
• Involves two divisions.

Question 4

What is asexual reproduction?

Answer 4

Asexual reproduction involves only one parent and no fusion of gametes. There is no mixing of genetic information. This leads to genetically identical offspring (clones). Only mitosis is involved.

Question 5

Why is meiosis important for sexual reproduction ? (2)

Answer 5

• It increases genetic variation.
• It ensures that the zygote formed at fertilisation is diploid (cell that contain two copies of each chromosome.)

Question 6

Describe fertilisation and its resulting outcome

Answer 6

Gametes join together to restore the normal number of chromosomes and the new cell then divides by mitosis (which increases the number of cells).
As the embryo develops, cells differentiate.

Question 7

What is DNA?

Answer 7

A double-stranded polymer of nucleotides, wound to form a double helix.

The genetic material of the cell found in its nucleus.

Question 8

Define genome

Answer 8

The entire genetic material of an organism

Question 9

Why is understanding the human genome important?

Answer 9

The whole human genome has been studied and is important for the development of medicine in the future.

• Searching for genes linked to different types of disease.
• Understanding and treating inherited disorders.
• Tracing human migration patterns from the past.

Question 10

What is a chromosome ?

Answer 10

A long, coiled molecule of DNA that carries genetic information in the form of genes.

Question 11

How many chromosomes do human body cells have ?

Answer 11

46 chromosomes (23 pairs)

Question 12

How many chromosomes do human gametes have

Answer 12

23 chromosomes

Question 13

Define gene

Answer 13

A small section of DNA that codes for a specific sequence of amino acids.

Question 14

What are alleles ?

Answer 14

Different versions of the same gene

Question 15

Wht is a dominant allele ?

Answer 15

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

Question 16

What is a recessive allele?

Answer 16

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

Question 17

What is meant when an organism is homozygous?

Answer 17

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

Question 18

What is meant when an organism is heterozygous?

Answer 18

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

Question 19

What is the genotype ?

Answer 19

The genes present for a trait

Question 20

What is the phenotype ?

Answer 20

The visible characteristic

Question 21

How are dominant alleles represented in a punnett square?

Answer 21

They are represented using uppercase letters.

Question 22

How are recessive alleles represented in a punnett square?

Answer 22

They use the lowercase version of the same letter as the dominant allele.

Question 23

What is the problem with single gene crosses?

Answer 23

Most characteristics are controlled by multiple alleles rather than just one.

Question 24

What is an inherited disorder?

Answer 24

A disorder caused by the inheritance of certain alleles.

Question 25

Give 2 examples of inherited disorders

Answer 25

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

Question 26

How are embryos screened for inherited disorders ?

Answer 26

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 27

What are the ethical issues concerning embryo screening?

Answer 27

• 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 28

What are the economic issues concerning embryo screening ?

Answer 28

• 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 29

What are the social issues concerning embryo screening ?

Answer 29

• 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 30

What is gene therapy ?

Answer 30

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

Question 31

What are the ethical issues concerning gene therapy ?

Answer 31

• 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 32

What are sex chromosomes ?

Answer 32

A pair of chromosomes that determine sex:
• Males have an X and a Y chromosome
• Females have two X chromosomes

Question 33

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

Answer 33

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

Question 34

What is a sex-linked characteristic ?

Answer 34

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

Question 35

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

Answer 35

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

Question 36

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

Answer 36

• 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 :: only one recessive allele is required to produce the recessive phenotype in males.

Question 37

What happens during the first stage of meiosis?

Answer 37

• Chromosome pairs line up along the cell equator.
• The pair of chromosomes are separated and move to opposite poles of the cell (the side to which each chromosome is pulled is random, creating variation).
• Chromosome number is halved.

Question 38

What happens during the second stage of meiosis?

Answer 38

• Chromosomes line up along the cell equator.
• The chromatids are separated and move to opposite poles of the cell.
• Four unique haploid gametes are produced.