Patterns of inheritance

Question 1

Continuous variation

Answer 1

when the individuals in a population vary within a range- there are no distinct categories
e.g. height, weight

Question 2

Discontinuous variation

Answer 2

when there are 2 or more distinct categories where an individual falls into 1 only- there are no intermediates
e.g. blood types, tongue-rolling ability

Question 3

Phenotypic variation

Answer 3

variation in an organism’s phenotypes. this is influenced by different factors like genotypes, environment, or both combined.

Question 4

What makes up an organism’s genotype?

Answer 4

their genes and alleles

Question 5

Individuals of the same species have the same genes but different?

Answer 5

alleles

Question 6

How can differences in genotypes occur?

Answer 6

1- sexual reproduction leads to variation in genotypes within a species
2- meiosis makes gametes with unique assortment of alleles through crossing-over and independent assortment of chromosomes
3- a random fusion of gametes during fertilisation

Question 7

What are inherited characteristics that show continuous variation usually influenced by?

Answer 7

many genes

Question 8

What are inherited characteristics controlled by many genes called?

Answer 8

Polygenic

e.g. human skin colour

Question 9

What are inherited characteristics that show discontinuous variation usually influenced by?

Answer 9

only one gene or a small no. of genes.

Question 10

What are characteristics controlled by only one gene called?

Answer 10

Monogenic

e.g. violet flower colour (either coloured or white)

Question 11

Give examples of the environment causing phenotypic variation.

Answer 11

1- Etiolation: plants grow abnormally long and spindly due to lack of light
2- Chlorosis: plants don’t produce enough chlorophyll and turn yellow due to factors like lack of magnesium in the soil.

Question 12

What type of variation does phenotypic variation controlled by both G+E show?

Answer 12

continuous variation

Question 13

Give examples of both genotype and the environment causing phenotypic variation.

Answer 13

1- Body mass in animals
2- Height of pea plants- they come tall or dwarf (genetic) but it can vary due to factors like light intensity and water availability.

Question 14

What is a gene?

Answer 14

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

Question 15

What is an allele?

Answer 15

different version of a gene

Question 16

Why do we have 2 alleles of each gene?

Answer 16

We inherit one copy of each chromosome of a pair from our parents.

Question 17

What is a locus?

Answer 17

A fixed position where the allele of each gene is found.

Question 18

What are codominant alleles?

Answer 18

alleles that are both expressed as neither are recessive

Question 19

What do genetic diagrams show?

Answer 19

the possible genotypes of offspring

Question 20

What is the expected ratio for monogenic crosses?

Answer 20

3:1

Question 21

What is dihybrid inheritance?

Answer 21

inheritance of 2 characteristics which are controlled by diff. genes. each of the 2 genes have different alleles.

Question 22

What is the expected ratio for a dihybrid cross of 2 heterozygous parents?

Answer 22

9:3:3:1

Question 23

What does it mean if a characteristic is sex-linked?

Answer 23

the allele it codes for is located on a sex chromosome

Question 24

Why are most genes on the sex chromosomes found on the X chromosome?

Answer 24

the Y chromosome is smaller and carries fewer genes

Question 25

Why are males more likely to express sex-linked genes even if its recessive?

Answer 25

they only have one allele for sex-linked genes.

Question 26

What are 2 examples of genetic disorders caused by faulty alleles on the sex chromosomes?

Answer 26

colour blindness and haemophilia

Question 27

What do we call males since they have 2 diff. kinds of sex chromosomes?

Answer 27

heterogametic

Question 28

What do we call females since they have one kind of sex chromosome?

Answer 28

homogametic

Question 29

Why is colour blindness rarer in women?

Answer 29

they need 2 copies of the recessive allele, while males need one.

Question 30

What is an autosome?

Answer 30

a chromosome that isn’t a sex chromosome

Question 31

Why are genes on the same autosome said to be linked?

Answer 31

they stay together during the independent assortment of chromosomes in meiosis 1 and their alleles are passed on to the offspring together.

Question 32

What problem could arise with linked autosomal genes?

Answer 32

crossing over could split them up first.

Question 33

The closer together 2 genes are on the autosome…

Answer 33

the more closely they are linked as crossing over is less likely to split them up

Question 34

What is the expected ratio of 2 autosomally linked genes in a dihybrid cross?

Answer 34

it would be expected to be 9:3:3:1 but is actually like a monogenic cross between 2 hetero parents, 3:1 as the genes are inherited together. a higher proportion of the offspring will have their parents’ genotype and phenotype.

Question 35

What is epistasis?

Answer 35

when the allele of one gene blocks the expression of the alleles of other genes. they interact to form the phenotype.

Question 36

Epistasis- widow’s peak example

Answer 36

a widow’s peak is controlled by one gene and baldness is controlled by others. if you have the alleles that code for baldness, the one for the widow’s peak isn’t expressed as you have no hair. the baldness genes are epistatic to the widow’s peak gene as it masks its expression.

Question 37

Epistasis- flower colour example

Answer 37

it is controlled by 2 genes. gene 1 codes for a yellow pigment (Y is the dom. yellow allele) and gene 2 codes for an enzyme that turns the yellow pigment orange (R is the dom. orange allele). if you don’t have the Y allele, having the R allele doesn’t matter as the flower will be colourless. gene1 is epistatic to gene 2.

Question 38

What does the phenotypic ratio from a dihybrid cross with an epistatic allele depend on?

Answer 38

it depends on whether the epistatic allele is recessive or dominant

Question 39

What does it take for a recessive epistatic allele to make the expression of other genes?

Answer 39

2 of them