Unit 2.4 Variation and Inheritance

Question 1

Species

Answer 1

Members of a species can interbreed to produce fertile offspring.

They are similar but not identical to each other.

They show variation as a result of sexual reproduction.

Question 2

Continuous variation

Answer 2

Controlled by many genes (polygenic).

Characteristics controlled by many genes show a range of values, eg. height, leaf size

There are no distinct groups.

Question 3

How is data showing continuous variation displayed?

Answer 3

Histogram or line graph

Question 4

Discrete (discontinuous) variation

Answer 4

Characteristics are controlled by a single gene and fall into distinct groups.

eg. tongue rolling - you are a roller, or a non-roller and nothing in between.

Other examples include blood type and hitchhikers thumb.

Question 5

How is data showing discrete variation displayed?

Answer 5

Bar chart

Question 6

Normal distribution

Answer 6

The pattern produced when you plot a histogram of data showing continuous variation. eg. height

Most values are average, with few extremes.

The curve is ‘bell shaped’.

Question 7

Monohybrid cross

Answer 7

Used to study the inheritance of characteristics that are controlled by a single gene and show discrete variation.

Alleles from 2 parents are combined and are used to predict the characteristics of the offspring.

Question 8

Gene

Answer 8

A section of DNA that carries the instructions for building a protein.

Genes are found on chromosomes.

Question 9

Alleles

Answer 9

The different versions of a gene.

They do the same job (eg. eye colour gene) but have a slightly different base sequence, so produce a slightly different protein.

Alleles for the eye colour gene are blue, green and brown.

Question 10

Homologous chromosomes

Answer 10

The 2 matching chromosomes in a pair.

One came from each parent.

They carry the same gene at the same location, but each one may have a different allele.

Question 11

Genotype

Answer 11

A 2 letter code describing which alleles have been inherited form each parent.

eg. BB indicates that a B allele came from each parent; Bb indicates that B came from one parent and b from the other.

Question 12

Alleles - capital letters

Answer 12

A capital letter indicates a dominant allele. eg. B = brown

Question 13

Alleles - lower case letters

Answer 13

A lower case letter indicates a recessive allele. eg. b = blue

Question 14

Homozygous

Answer 14

If the 2 alleles in a homologous pair are the same, the organism is said to be homozygous.

eg. BB or bb

Question 15

Heterozygous

Answer 15

If the 2 alleles in a homologous pair are different, the organism is said to be heterozygous.

eg. Bb

The characteristic that is present depends on which allele is dominant.

Question 16

Phenotype

Answer 16

Describes how a gene is expressed.

It is often (but not always) appearance.

eg. blue eyes, brown fur, blood group A.

Phenotype is determined by 2 alleles working together. The dominant one is expressed, the recessive one may be present but hidden.

Question 17

F1 generation

Answer 17

The 1st generation of offspring, produced by combining the alleles of 2 homozygous parents (P)

Question 18

F2 generation

Answer 18

The offspring produced when the alleles of 2 members of the F1 generation are combined.

Question 19

Punnett square

Answer 19

A grid used to show the results of a monohybrid cross.

Question 20

Phenotype ratio

Answer 20

Phenotype ratio shows the predicted proportion of each phenotype that should be produced eg. 3 purple : 1 white flowered.

This ratio is not always achieved due to random fertilisation and different survival rates of offspring.

Question 21

Genotype ratio

Answer 21

Genotype ratio shows the predicted proportion of each genotype that should produced eg. 1 BB : 2 Bb : 1 bb

This ratio is not always achieved due to random fertilisation and different survival rates of offspring.

Question 22

Backcross

Answer 22

Used to identify whether an original parent is homozygous or heterozygous.

If the unknown parent is crossed with another individual of known genotype (homozygous recessive) then offspring ratios can be used to identify the unknown parent.

Question 23

Family trees

Answer 23

Used to follow the inheritance of a characteristic between generations, by identifying the individuals that are homozygous recessive (bb).

Question 24

Genetic counselling

Answer 24

Family trees are used to advise people who are concerned about passing on an inherited disease to their children.

Carriers may not suffer from a disease if it is recessive, but it can be inherited.
eg. cystic fibrosis,

Some genetic diseases have dominant alleles eg. Huntington’s disease.