Patterns of inheritance Module 6

Question 1

What is continuous variation?

Answer 1

When the individuals within a population vary within a range eg.height

Question 2

What’s discontinuous variation?

Answer 2

When there are 2 or more distinct categories, each individual only falls into one of these catergories
eg. blood group

Question 3

How can variation be influenced by your genes?

Answer 3

Individuals of the same species have the same genes but different versions of them called alleles - making up it’s phenotype

Sexual reproduction leads to variation in genotypes within a species, as meiosis makes gametes with a unique assortment of alleles through crossing over and the independent assortment of chromosomes

The random fusion of gametes during fertilisation also increases the gentic variation in the offspring

This all leads to phenotypic variation

Question 4

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

Answer 4

Many genes so are known as polygenic

Question 5

Inherited characteristics that show discontinuous variation are usually influenced by?

Answer 5

One gene so are monogenic

Question 6

2 examples of environment affecting variation?

Answer 6

Etiolation- when plants go abnormally long and spindly because they aren’t getting enough light

Chlorosis - this is when plants don’t produce enough chlorophyll and turn yellow due to a lack of magnesium

Question 7

Definition of gene?

Answer 7

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

Question 8

Definition of an allele?

Answer 8

A different version of a gene, humans have 2 alleles of each gene, one from each parent

Question 9

Definition of a genotype?

Answer 9

The alleles an organism has, eg BB, Bb or bb

Question 10

Definition of a phenotype?

Answer 10

The characteristics the alleles produce

Question 11

Definition of a dominant allele?

Answer 11

An allele whose characteristic appears in the phenotype even when there’s only one copy, it’s shown by having a capital letter

Question 12

Definition of a recessive allele?

Answer 12

An allele whose characteristics only appear in the phenotype if 2 copies are present. Shown by a lower case letter

Question 13

Definition of codominant?

Answer 13

Alleles that are both expressed in the phenotype - neither one is recessive

Question 14

Definition of a locus?

Answer 14

The fixed position of a gene on a chromosome - alleles of a gene are found at the same locus on each chromosome in a pair

Question 15

Definition of a homozygote?

Answer 15

An organism that carries 2 copies of the same allele eg. BB or bb

Question 16

Definition of a heterozygote?

Answer 16

An organism that carries 2 different alleles eg. Bb

Question 17

Definition of a carrier?

Answer 17

A person carrying an allele which is not expressed in the phenotype, but can be passed on to the offspring

Question 18

Body cells of individuals have?

Answer 18

2 alleles of each gene

Question 19

Gametes have?

Answer 19

Only one allele of each gene

Question 20

How would you produce the monogenic cross for the alleles N normals wings and n little wings

The parents are NN and nn

Answer 20

Cross the parents so NN x nn

Write out each ones gametes alleles so they are going to be N N n n

Then put them in a punnet square to find the possible genotypes of the F1 offspring, going to Nn, Nn, Nn, Nn

Now found the gamete allels again, going to be N n N n

Punnet square it again, leaving you with the genotypes NN, Nn, Nn ,nn the F2 offspring

So phenotypic ratio will be 3:1 of normal wings and little wings

RATIO WILL ALWAYS BE 3:1

Question 21

Describe how to do the punnet square for sickle cell amenia when the alleles are codominant so neither one is recessive for 2 heterozygous parents (H(s)H(n)?

These parents will have been produced from crossing H(s)H(s) with H(n)H(n)

Answer 21

H(n)H(n) so homozygous for normal don’t have the disease so have normal haemoglobin

H(s)H(s) so homozygous for sickle anaemia so all their blood cells are sickle shaped

Some people have H(s)H(n) so have sickle cell trait, so have some normal haemoglobin and some sickle cell shaped - as the alleles are co dominant as both expressed in the phenotype

Do the cross by doing H(n)H(s) x H(n)H(s)

Find the gametes which will be H(n), H(s), H(n), H(s)

Cross them in a punnet square to get H(n)H(n) , H(n)H(s), H(n)H(s), H(s)H(s)

So 25% chance will be unaffected, 50% chance will have mixed traits and 25% chance will have sickle cell amenia

Question 22

Describe how to do inheritance when genes have multiple alleles such in blood groups and cross the parents with the genotypes I(a)I(o) and I(b)I(o)?

Answer 22

I(o) is the allele for blood group O
I(a) is the allele for blood group A
I(b) is the allele for blood group B

Allele I(O) is recessive and I(a) and I(b) are codominant

So will be crossing I(a)I(o) x I(b)I(o)

Gametes will be I(a), I(o), I(b), I(o)

Now when you cross the alleles you will get I(a)I(b), I(a)I(o), I(b)I(o), and I(o)I(o)

So the phenotypes will be AB, A, B and O as the O is recessive to the A and the B and the A and B are codominant

Question 23

What’s dihybrid inheritance?

Answer 23

The inheritance of 2 characteristics why are controlled by different genes

Question 24

How would you do a dihybrid cross for pea plants when

R  = round seed
r= wrinkled seed
Y = yellow seed
y = green seed

Answer 24

So we will be crossing the parents with the genotypes RrYy x RrYy in the F1 offspring, the parents are RRYY and rryy from the original)
Giving us the gametes, RY, Ry, rY, ry,RY, Ry, rY, ry

Then we produce a 4 x 4 punnet square which will produce us the phenotypic ratio of 9 yellow and round seeds, 3 Round and green seeds, 3 wrinkled and yellow seeds and 1 wrinkled and yellow seed

Question 25

For a monogenic cross, what parents are you crossing, what will be the phenotype in the F1 ratio, and the phenotypic ratio in the F2?

Answer 25

Parents crossing will be homozygous dominant x homozygous recessive

Phenotypic ratio in the F1 will be all heterozygous offspring eg Rr

Phenotypic ratio in F2 will be 3:1 dominant to recessive

Question 26

For a dihybrid cross, what parents are you crossing, what will be the phenotype in the F1 ratio, and the phenotypic ratio in the F2?

Answer 26

Parents are homozygous dominant x homozygous recessive (eg RRYY x rryy)

Phenotypic ratio in offspring will all be heterozygous (eg RrYy)

Phenotypic ratio in F2 will be 9:3:3:1

Dominant both: dominant 1st recessive 2nd : dominant 2nd recessive 1st : both recessive

Question 27

For a codominant cross, what parents are you crossing, what will be the phenotype in the F1 ratio, and the phenotypic ratio in the F2?

Answer 27

Parents are homozygous for one allele and homozygous for the other allele, eg H(n)H(n) x H(s)H(s)

Phenotypic ratio in the F1 will be all heterozygous H(n)H(s)

Phenotypic ratio in the F2 will be 1:2:1
Homozygous for 1 allele : heterozygous : homozygous for the other allele

Question 28

The genetic information for gender is carried on?

Answer 28

2 sex chromosomes

Question 29

What are the chromosomes for female and male?

Answer 29

XX = female
XY = male

Question 30

A characteristic is said to be sex linked when?

Answer 30

When the allele that codes for it is located on a sex chromosome

Question 31

Why are most genes only carried on the X chromosome?

Answer 31

The Y chromosome is smaller than the X chromosome, so can’t carry as many genes

Question 32

Why are males more likely to express recessive phenotypes?

Answer 32

They only have 1 X chromosome, so they often only have 1 allele for for sex linked genes, so they will express it even if it’s recessive

Question 33

How would you draw a genetic diagram for colour blindness
X(N) = normal vision
X(n) = colour blind
Y = no effect as dosen’t have an allele for it

With a carrier female and an unaffacted male?

Answer 33

Crossing X(N)X(n) (female) with X(N)Y

Gametes will be

X(N), X(n), X(N), Y

Cross them to get the offspring
X(N)X(N), X(N)Y, X(n)X(N), X(n)Y

So produces an unaffected male, an unaffected female, a carrier female, and a colour blind male

Question 34

What’s an autosome?

Answer 34

Any chromosome which isn’t a sex chromosome, so autosomal genes are located on autosomes

Question 35

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

Answer 35

Because they are on the same autosome they will stay together during the independent asortment of chromosomes in meiosis 1, and their alleles will be passed on to the offspring together- the only reason they won’t be together is if crossing over splits them up first

Question 36

The closer 2 genes are together on the autosome means?

Answer 36

The more closely they are said to be linked, as crossing over is less likely to split them up

Question 37

What’s the expected dihybrid phenotypic ratio if the chromosomes are autosomally linked?

Answer 37

3:1

Question 38

What does an epistatic gene do?

Answer 38

Masks the expression of another gene, so won’t result in the expected phenotypic ratio

Question 39

What ratio will you get for a dihybrid cross involving a recessive epistatic allele - so if it’s recessive homozygous it will mask the affect of the other gene?

Answer 39

9:3:4

At least one dominant from each : The epistatic gene being dominant and the other gene being recessive : The effect of both episatic alles being recessive

Question 40

What ratio will you get if the there’s a dominant epistatic allele?

Answer 40

12:3:1

Question 41

How do to a Chi-Squared test in genetics?

Answer 41

State the null hypothesis, and the alternate hypothesis

Draw the observed table
Then create an expected table by using the ratio required for cross, eg monogenic = 3:1

Work out the X^2 (chi squared value) using the formula of: Sum of all ((observed - expected)^2)/E

Compare it to the critical value will always be at 0.95 probability, degrees of freedom = number of phenotypes -1

Compare the critical value and the test value, if test value is higher there is evidence to reject H0, if it’s lower then there is evidence to accept H0