20.1 Variation And Inheritance

Question 1

Define continuous variation
E.g.?

Answer 1

A characteristic that can take any value within a range
- caused by genetic and environmental variation
- controlled by a number of genes (poly genetic)
- e.g. anima mass, leaf surface area, height

• displayed using a histogram

Question 2

Define discontinuous variation
E.g.?

Answer 2

A characteristic that can only appear in specific (discrete) values
- caused by, mostly, genetic variation
- controlled by one or two genes
- e.g. blood group, albinism, round or wrinkled pea shape

Displayed on a bar graph

Question 3

Define phenotypic variation

Answer 3

The variation in an organism’s phenotype (the characteristics it displays)
- phenotypic variation is influenced by genetic and environmental factors

Question 4

What causes phenotypic variation?

Answer 4

1) Genotype
- sexual reproduction leads to variation in genotype within a species
E.g. meiosis (independent assortment and crossing over) and the random fusion of gametes during fertilisation increases genetic variation.
Differences in genotype result in phenotypic variation

2) the environment
the climate, food, lifestyle
E.g. etiolation - this is when plants grow abnormally long and spindly because they’re not getting enough light
Chlorosis- this is when plants don’t produce enough chlorophyll and turn yellow. It’s caused by several environmental factors e.g. lack of magnesium in soil

3) genotype and the environment
Genotype tends to influence the characteristic an organism is born with, but the environmental factors tends to influence how the some characteristics develop
Phenotypic variation usually influence by both and shows continuous variation e.g. body mass (influences by genes and diet)

Question 5

Define gene

Answer 5

A sequence of bases on a DNA molecules that codes for protein which results in a characteristic

Question 6

Define allele

Answer 6

Different versions of a gene (so code for different versions of the same characteristic).
- the order of bases in each allele is slightly different
- e.g. allele of eye colour (could be brown or blue)

• most plants and animals have two alleles of each gene, one from each parent
  Thats because we inherit one copy of each chromosome of a pair from our parents
  The allele of each gene is found at a sized position called the locus on each chromosome in a pair

Question 7

Define genotype

Answer 7

The genetic makeup of an organism (the alleles that it has)

Question 8

Difference between homozygous and heterozygous genotype

Answer 8

• if an organism carries two copies of the same allele it’s homozygous (could be homozygous recessive or homozygous dominant)
• if an organism carries two different alleles for a characteristic it’s heterozygous

Question 9

Difference between recessive and dominant alleles?

Answer 9

• dominant allele will always be expressed (even if only one copy is present), shown by capital letter
• recessive allele only expressed in phenotype if two copies are present

Question 10

Define codominant alleles

Answer 10

Some alleles are both expressed in the phenotype because neither one is recessive

E.g. horses can have alleles for white hair or coloured hair. Neither allele is recessive, so a horse with one copy of each allele will have a coat with a mixture of white hairs and coloured hairs

Question 11

Define carrier

Answer 11

A carrier is a person carrying an allele which is not expressed in the phenotype but can be passed onto their offspring

E.g. cystic fibrosis is an inherited recessive disease, so needs two copies of that allele to be expressed. Is someone only has one allele for cystic fibrosis and one normal allele they would only be a carrier of the disease