Section 3

Question 1

what is a gene?

Answer 1

is a section of DNA that codes for a protein

Question 2

what does homozygous mean?

Answer 2

means that a person has two of the same alleles whether two recessive alleles e.g. dd or two dominant alleles e.g. DD

Question 3

what does heterozygous mean?

Answer 3

means that a person has one of each allele e.g. Dd

Question 4

what is a dominant allele?

Answer 4

a dominant allele is an allele which is always expressed if present

Question 5

what is a recessive allele?

Answer 5

a recessive allele is an allele which is only expressed if there are no dominant allele so dd

Question 6

What does co-dominant mean?

Answer 6

co-dominant means that there are two different alleles which are both expressed in the genotype e.g. BW and both contribute to the phenotype

Question 7

What is a genotype?

Answer 7

A genotype is the genetic makeup of an organism e.g. Bb or BB or bb

Question 8

What is a phenotype?

Answer 8

A phenotype is an expression of the genotype e.g. B is for the dominant allele for brown hair.

Question 9

Name the four bases:

Answer 9

1. Adenine
2. Thymine
3. Guanine
4. Cytosine

Question 10

name five differences between mitosis and meiosis:

Answer 10

Mitosis:

• happens in body cells
• forms diploid cells (46 chromosomes)
• forms genetically identical cells
• one cell division
• produces two daughter cells

Meiosis

• happens only in the ovaries or the testes
• forms haploid gametes (23 chromosomes)
• forms non-identical cells
• two cell divisions
• four daughter cells

Question 11

What are the points of the markscheme for natural selection? (7)

Answer 11

• variation in the population
• some are better adapted than others i.e. selective advantage
• these animals will survive
• then they will reproduce
• they pass on their advantages alleles to the next generation (this continues for many generations)
• the less advantaged animals will die out and become a minority
• this is called evolution