Patterns of inheritance

Question 1

What is continuous variation?

Answer 1

when the individuals in a population vary within a range
there are no distinct categories
it can be shown by continuous data
quantitative

Question 2

What is discontinuous variation?

Answer 2

when there are two or more distinct categories
each individual falls into only one of these categories
there are no intermediates
qualitative

Question 3

define genotype

Answer 3

the alleles that an organism has

Question 4

define phenotype

Answer 4

the characteristics that the alleles produce

Question 5

What is phenotypic variation?

Answer 5

the variation in organisms phenotype i.e the characteristics it displays

Question 6

what factors are phenotypic variation influenced by?

Answer 6

1) genotype- individuals of the same species have the same genes but different alleles
crossing over, independent assortment of chromosomes & random fusion of gametes during fertilisation increases genetic variation
2) the environment- variation can be caused by differences in the environment e.g. climate, food, lifestyle
3) genotype and the environment- genotype tends to influence the characteristics an organism is born with & environmental factors can influence how the characteristics develop- usually shows continuous variation

Question 7

define monogenic

Answer 7

characteristics controlled by only one gene

Question 8

define polygenic

Answer 8

characteristics influenced by many genes

Question 9

define a gene

Answer 9

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

Question 10

what is an allele?

Answer 10

a version of a gene

each allele codes for different versions of the same characteristic

Question 11

what is a locus?

Answer 11

the allele of each gene is found a fixed position called a locus

Question 12

define homozygous and heterozygous

Answer 12

homozygous- is when an organism carries two copies of the same allele (BB,bb)
heterozygous- is when an organism carries two different alleles (Bb,bB)

Question 13

define dominant and recessive alleles

Answer 13

dominant alleles- those whose characteristics appear in the phenotype even when there’s only one copy
recessive alleles- those whose characteristics only appear in the phenotype if two copies are present

Question 14

define codominant alleles

Answer 14

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

Question 15

define a carrier

Answer 15

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

Question 16

what is monogenic inheritance?

Answer 16

it’s the inheritance of a characteristic controlled by a single gene
all offspring produced are heterozygous as one allele is inherited from each parent
usually when doing a monogenic cross with two heterozygous parents you get a 3:1 ratio of dominant:recessive characteristics

Question 17

How do you do a monogenic inheritance of codominant alleles?

Answer 17

when both alleles are expressed in the phenotype and neither one is recessive e.g. sickle cell anaemia
cross produces a 1:2:1 phenotypic ratio of unaffected homozygous: heterozygous: disorder homozygous

Question 18

how do you do a multiple allele cross?

Answer 18

e.g. blood group
any offspring could have one of four different blood groups A, B,O,or AB
expected phenotypic ratio is 1:1:1:1

Question 19

What are dihybrid crosses?

Answer 19

inheritance of two characteristics which are controlled by different genes
shows the likelihood of offspring inheriting certain combinations of the two characteristics from particular parents
usually when you do a dihybrid cross with two heterozygous parents you get 9:3:3:1 phenotypic ratio of dominant both: dominant first, recessive second: recessive first, dominant second: recessive both

Question 20

What are sex linked characteristics?

Answer 20

females (XX) & males (XY)
the alleles that code for characteristic are located on a sex chromosome
the Y chromosome smaller than the X chromosome and carries few genes so most genes on the sex chromosomes are only carried on the X
females need two copies of the recessive allele to be colour blind & males only need one copy
females with one copy of the recessive allele are carriers

Question 21

why are males more likely to show phenotypes for genes that are sex linked?

Answer 21

as males only have one X chromosome they often only have one allele for sex linked genes so they only have one copy so they express the characteristic even if it’s recessive
this makes males more likely than females to show recessive phenotypes for genes that are sex linked

Question 22

what are some genetic disorders caused by faulty alleles located on sex chromosomes?

Answer 22

colour blindness and haemophilia
alleles are carried on the X chromosome and so its called x-linked disorders
y linked disorders do exist but are less common

Question 23

define autosome

Answer 23

any chromosome that isn’t a sex chromosome

Question 24

What is autosomal linkage?

Answer 24

genes on the same autosome are said to be linked because they stay together during the independent assortment of chromosomes and the allele will be passed onto the offspring together
the closer together two genes are on the autosome the more closely they are said to be linked this is because crossing over is less likely to split them up
if two genes are autosomally linked, you won’t get the expected phenotypic ratio

Question 25

What is epistasis?

Answer 25

when the allele of one gene masks/blocks the expression of the alleles of the other gene

Question 26

what is the difference between recessive epistatic alleles and dominant epistatic alleles?

Answer 26

recessive: two copies of it will mask the expression of the other gene
cross a homozygous recessive with homozygous dominant will produce 9:3:4 ratio of dominant both: dominant epistatic, recessive other: recessive epistatic
dominant: having at least one copy of it will mask the expression of the other gene
crossing a homozygous recessive with a homozygous dominant produce 12:3:1 ratio of dominant epistatic: recessive epistatic, dominant other: recessive both

Question 27

What is the chi-squared test?

Answer 27

test used to see if the results of the experiment support a theory
following the expected and the observed results you produce a null hypothesis
to find out if the results were significant you first need to calculate the chi-squared value and compare it to the critical value
if calculated value is MORE than critical value: REJECT null hypothesis
if calculated value is LESS than critical value: ACCEPT null hypothesis

Question 28

How would you write a conclusion for a chi-squared test?

Answer 28

The chi-squared value of 2.7 is smaller than the critical value of 3.84
this means that there’s no significant difference between observed and expected results
we fail to reject the null hypothesis
the theory is supported