Module 6 Section 2: 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
e.g. humans can be any height within a range not just tall or short

Question 2

Example of continuous variation

Answer 2

Height
Waist circumference
Fur length

Question 3

What is discontinuous variation

Answer 3

When there are two or more distinct categories
Each individual falls into only one of these categories
There are no intermediates

Question 4

Example of discontinuous variation

Answer 4

Blood group (ABO groups or Rhesus + or - grouping)
Violets (flowers) can either be coloured or white

Question 5

How is variation influenced by genes

Answer 5

Different species have different genes.
Individuals of the same species have the same genes but different versions of them (alleles).
The genes and alleles an organism has make up its genotype.
Differences in genotypes can then affect the phenotype

Question 6

How do genotypes vary via sexual reproduction

Answer 6

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 genetic variation in the offspring

Question 7

What do differences in genotypes create (with example)

Answer 7

Results in variation in phenotype - the characteristics displayed by an organism.
Variation in phenotype is also referred to as phenotypic variation
E.g. 4 different blood group alleles which create 4 different blood groups

Question 8

How many genes are inherited characteristics that show continuous variation usually influenced by

Answer 8

Continuous variation influenced by many genes
These are polygenic
E.g. human skin colour is polygenic - comes from lots of different shades of colour

Question 9

How many genes are inherited characteristics that show discontinuous variation usually influenced by

Answer 9

Discontinuous variation is usually influenced by one gene (or small number of genes)
These are monogenic
E.g. violet flower colour is monogenic, either white or coloured

Question 10

How can variation be influenced by the environment

Answer 10

Variation can also be caused by differences in the environment, e.g. climate, food, lifestyle.
Characteristics controlled by environmental factors can change over an organism’s life

Question 11

Examples of how variation be influenced by the environment

Answer 11

Etiolation
This is when plants grow abnormally long and spindly because they’re not getting enough light (due to auxins)
Chlorosis:
This is when plants don’t produce enough chlorophyll and turn yellow
It’s caused by several environmental factors, e.g. a lack of magnesium in soil

Question 12

How can variation be influenced by genetics and the environment

Answer 12

Genetic factors determine genotype and the characteristics an organism’s born with
But environmental factors can influence how some characteristics develop.
Most phenotypic variation is caused by the combination of genotype and environmental factors.
Phenotypic variation influenced by both usually shows continuous variation.

Question 13

Examples of how variation can be influenced by genetics and the environment

Answer 13

Height of pea plants:
Pea plants come in tall and dwarf forms (discontinuous variation)
This is determined by genotype.
The exact height of the tall and dwarf plants varies (continuous variation) because of environmental factors (e.g. light intensity and water availability affect how tall a plant grows).

Body mass in animals
Partly genetic, but also strongly influenced by environmental factors like diet.
E.g. if your diet doesn’t contain enough of the right nutrients, your body mass is likely to be lower than that determined by your genes.
Body mass varies within a range so it’s continuous variation.

Question 14

Gene

Answer 14

A sequence of bases on a DNA molecule that codes for a protein (polypeptide), which results in a characteristic, eg. a gene for eye colour

Question 15

Allele

Answer 15

A different version of a gene.
Most plants and animals, including humans, have two alleles of each gene, one from each parent.
The order of bases in each allele is slightly different - they code for different versions of the same characteristic.
They’re represented using letters, eg. the allele for brown eyes (B) and the allele for blue eyes (b).

Question 16

Genotype

Answer 16

The alleles an organism has, e.g. BB, Bb or bb for eye colour.

Question 17

Phenotype

Answer 17

The characteristics the alleles produce, eg. brown eyes.

Question 18

Dominant

Answer 18

An allele whose characteristic appears in the phenotype even when theré’s only one copy.
Dominant alleles are shown by a capital letter.
Eg the allele for brown eyes (B) is dominant - if a person’s genotype is Bb or BB, they’ll have brown eyes

Question 19

Recessive

Answer 19

An allele whose characteristic only appears in the phenotype if two copies are present.
Recessive alleles are shown by a lower case letter.
Eg. the allele for blue eyes (b) is recessive - if a person’s genotype is bb, they’ll have blue eyes.

Question 20

Codominant

Answer 20

Alleles that are both expressed in the phenotype
Neither one is recessive and both are equally dominant
E.g. with the black and white patches in a cow, the alleles cannot mix colours to make grey, they are separate

Question 21

Locus

Answer 21

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 22

Homozygote

Answer 22

An organism that carries two copies of the same allele, e.g. BB or bb

Question 23

Heterozygote

Answer 23

An organism that carries two different alleles, e.g. Bb.

Question 24

Carrier

Answer 24

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

Question 25

How is the genotype for the offspring produced

Answer 25

The body cells of individuals have two alleles for each gene.
Gametes (sex cells) contain only one allele for each gene
When carriers from two parents fuse together, the alleles they contain form the genotype of the offspring produced

Question 26

What can genetic diagrams be used for

Answer 26

Genetic diagrams can be used to predict the genotypes and phenotypes of the offspring produced if two parence are crossed (bred)

Question 27

How to show dominant and recessive alleles in genetic diagrams

Answer 27

Dominant: CAPITAL LETTER
Recessive: lower case letter

Question 28

How to set out punnet square correctly

Answer 28

Write out parent phenotype and genotype
Work out the alleles the gametes would have
Cross the parents gametes to show possible genotypes
Work out %s and ratios of genotypes

Question 29

What are the characteristics of monogenic inheritance

Answer 29

Single gene locus, multiple alleles
Patterns of dominance, recessiveness, codominance or multiple genes acting epistatically

Question 30

What are the characteristics of polygenic inheritance

Answer 30

Multiple genes influencing a particular characteristic
Multiple genes with multiple alleles
Tend to show normal distribution

Question 31

What is incomplete dominance

Answer 31

Where a dominant allele does not completely mask the effects of a recessive allele
E.g. in roses there are separate alleles that code for enzymes which create red and white petals, these alleles can occur together to mix the colours and make pink petals

Question 32

What is monohydbrid inheritance

Answer 32

Inheritance patterns for a single gene locus (location on a gene on the chromosome)

Question 33

What would you called these genotypes
GGYY
Ggyy

Answer 33

GGYY: homozygous dominant at both gene loci
Ggyy: Heterozygous dominant at G locus, homozygous recessive at Y locus

Question 34

How is sickle cell anaemia show codominance

Answer 34

People who are homozygous for normal haemoglobin (H^N H^N) don’t have the disease.
People who are homozygous for sickle haemoglobin (H^S H^S) have sickle-cell anaemia - all their blood cells are sickle-shaped (crescent-shaped).
People who are heterozygous (H^N H^S) have an inbetween phenotype, called the sickle-cell trait
Have some normal haemoglobin and some sickle haemoglobin
The two alleles are codominant because they’re both expressed in the phenotype

Question 35

Show the genetic cross for two adults which are heterozygous with sickle cell trait (codominance)

Answer 35

Question 36

What are the alleles for ABO blood type

Answer 36

Question 37

How does ABO blood grouping show codominance

Answer 37

I^O is recessive
I^A and I^B are codominant
People with genotype I^A I^B will have blood group AB

Question 38

Show genetic cross between person with heterozygous blood type A and heterozygous blood type B

Answer 38

Question 39

What is dihybrid inheritance

Answer 39

The inheritance of two characteristics which are controlled by two different genes
Worked out using a dihybrid cross

Question 40

Work out inheritance for heterozygous pea plant parents with round and yellow seeds

Answer 40

Question 41

What is the phenotypic ratio

Answer 41

The ratio of different phenotypes in offspring
F1 and F2 predicted using genetic diagrams

Question 42

Table of F1 and F2 phenotype ratio with differing homozygous parents for monogenic, dihybrid and codominance

Answer 42

Question 43

What can change the phenotypic ratio from the normal

Answer 43

Can be because of sex linkage, autosomal linkage or epistasis

Question 44

What are the genotypes of the different sexes

Answer 44

The genetic information for biological sex is carried on two sex chromosomes.
In mammals, females have two X chromosomes (XX) and males have one X and one Y chromosome (XY)

Question 45

What is sex linkage

Answer 45

A characteristic is said to be sex-linked when the allele that codes for it is located on a sex chromosome

Question 46

Why are males more likely to get genetic disorders

Answer 46

The Y chromosome is smaller than the X chromosome and carries fewer genes.
So most genes on the sex chromosomes are only carried on the X chromosome (called X-linked genes).
As males only have one X chromosome, they often only have one allele for sex-linked genes.
So because they only have one copy, they express the characteristic of this allele even if it’s recessive.
This makes males more likely than females to show recessive phenotypes for genes that are sex-linked

Question 47

Examples of sex linked disorders

Answer 47

Genetic disorders caused by faulty alleles on sex chromosomes include colour blindness and haemophilia.
The faulty alleles for both of these disorders are carried on the X chromosome - they’re called X-linked disorders

Question 48

How is colourblindness a sex linked disorder

Answer 48

Colour blindness is a sex-linked disorder caused by a faulty allele carried on the X chromosome.
As its sex-linked both the chromosome and the allele are represented in the genetic diagram, e.g. X^n where X represents the X chromosome and n the faulty allele for colour vision.
The Y chromosome doesn’t have an allele for colour vision so is just represented by Y.
Females would need two copies of the recessive allele to be colour blind, while males only need one copy
This means colour blindness is much rarer in women than men.

Question 49

Draw the genetic cross of colourblindness for the offspring of a male and female where male is heterozygous healthy and female is heterozygous carrier
N is normal vision, n is faulty colour vision

Answer 49

3:1 ratio of offspring without colour blindness : offspring with colour blindness
Only their male offspring are at risk of being colour-blind.
So you can also say that there’s a predicted 2:1:1 ratio of female offspring without colour-blindness:male offspring without colourblindness:male offspring with colourblindness

Question 50

Evidence for how sex linkage can change the traditional phenotypic ratio (using colourblindness with healthy male and carrier female and colourblind male and carrier female)

Answer 50

Normal phenotypic ratio for a female carrier and healthy male is 3:1 in F2 (normal monogenic ratio)
This ratio will change if a female carrier (X^N X^n) and a colourblind male (X^n Y) have children.
The predicted ratio will then be 1: 1 - of offspring with colour-blindness : offspring without colour-blindness.
The ratio will be the same for offspring of each sex.
You only end up with this predicted ratio for a monogenic F2 cross with a sex-linked characteristic

Question 51

What is an autosome and what is an autosomal gene

Answer 51

Autosome is the name for any chromosome that isn’t a sex chromosome
Autosomal genes are the genes located on the autosomes

Question 52

Why are genes on the same autosome linked

Answer 52

Genes on the same autosome are said to be linked
Because they’re on the same autosome they’ll stay together during the independent assortment of chromosomes in meiosis I
Alleles will be passed on to the offspring together.
The only reason this won’t happen is if crossing over splits them up first
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.

Question 53

When does independent assortment and crossing over happen

Answer 53

Crossing over (prophase 1)
Independent assortment of chromosomes at metaphase 1
Independent assortment of sister chromatids at metaphase 2

Question 54

What happens to the phenotypic ratio if the two genes are autosomally linked

Answer 54

The phenotypic ratio won’t be what is expected

Question 55

What may happen to the F2 ratio on a dihybrid cross if the genes turn out to be autosomally linked

Answer 55

You would expect a 9: 3: 3: 1 ratio.
Instead, the phenotypic ratio is more likely to be that expected for the F2 generation of a monohybrid cross (3: 1)
Because the two autosomally-linked alleles are inherited together.
This means that a higher proportion of the offspring will have their parents’ genotype and phenotype

Question 56

What is epistasis

Answer 56

A gene is said to be epistatic when it’s presence modifies or suppresses (masks) the effect of a gene at another locus (hypostatic gene)
Sometimes referred to as modifying genes or inhibiting genes

Question 57

Where on the genes does epistasis refer to

Answer 57

Epistasis describes an interaction between genes at different loci
Affecting a single phenotypic trait

Question 58

What is recessive epistasis

Answer 58

The presence of homozygous recessive alleles at the first locus prevents the expression of another allele at a second locus
The hypostatic genes are essentially redundant if the epistatic locus is homozygous recessive

Question 59

What is the phenotypic ratio of recessive epistasis of a homozygous recessive parent and a homozygous dominant parent in the F2 generation for a dihybrid inheritance

Answer 59

You will get a 9: 3: 4 phenotypic ratio of dominant both: dominant epistatic recessive at other locus : recessive epistatic IN THE F2 GENERATION

Question 60

How is baldness and widows peak an example of epistasis in humans

Answer 60

In humans a widow’s peak is controlled by one gene and baldness by others.
If you have the alleles that code for baldness, it doesn’t matter whether you have the allele for a widow’s peak or not, as you have no hair.
The baldness genes are epistatic to the widow’s peak gene, as the baldness genes mask the expression of the widow’s peak gene

Question 61

How is flower pigment an example of epistasis in plants

Answer 61

Flower pigment in a plant is controlled by two genes.
Gene 1 codes for a yellow pigment
(Y is the dominant yellow allele) and gene 2 codes for an enzyme that turns the yellow pigment orange (R is the dominant orange allele).
If you don’t have the Y allele it won’t matter if you have the R allele or not as the flower will be colourless.
Gene 1 is epistatic to gene 2 as it can mask the expression of gene 2.

Question 62

Are crosses of epistatic genes the same as normal crosses

Answer 62

Crosses involving epistatic genes don’t result in the expected phenotypic ratios given
e.g. if you cross two heterozygous orange flowered plants (YyRr) you wouldn’t get the expected 9:3:3:1 phenotypic ratio for a normal dihybrid cross

Question 63

Mechanisms behind how recessive epistasis results in differing phenotypes in pigmented plants

Answer 63

If enzyme B and enzyme A work then substance goes from colourless to pink to purple
If enzyme A works but alleles code for an enzyme B that doesn’t work then only pink pigment is produced
If enzyme B works but alleles code for an enzyme A that doesn’t work then no pigment is produced as the pigments can’t even turn pink first and the substance stays colourless

Question 64

Draw the genetic cross for recessive epistasis between homozygous dominant parent at gene loci Y and R and homozygous recessive parent at gene loci Y and R
If a plant is homozygous recessive for the epistatic gene (yy) then it will be colourless, masking the expression of the orange gene

Answer 64

Question 65

What is dominant epistasis

Answer 65

A dominant allele at the epistatic gene ‘masks’ the hypostatic gene
(even if there is a dominant allele present at the hypostatic gene locus)

Question 66

Phenotypic ratio of F2 generation when crossing a homozygous recessive parent with a homozygous dominant parent for dominant epistasis for dihybrid inheritance

Answer 66

Will produce a 12: 3: 1 phenotypic ratio for dihybrid inheritance of dominant epistatic: recessive epistatic dominant other : recessive both in the F2 GENERATION

Question 67

Mechanisms behind how dominant epistasis results in differing phenotypes

Answer 67

Dominant allele codes for enzyme that doesn’t work so pigments can’t be produced (as opposed to recessive gene is recessive epistasis)
If enzyme ii and enzyme C work then substance goes from colourless to colourless to coloured
If enzyme ii works but alleles code for an enzyme C that doesn’t work (by having dominant allele) then only 2nd colourless pigment is produced
If enzyme C works but alleles code for an enzyme ii that doesn’t work (by having a dominant allele) then no pigment is produced and the substance stays colourless

Question 68

Draw the genetic cross for homozygous recessive parent at gene loci W and Y and homozygous dominant parent at gene loci W and Y for 2 generations:
Squash colour is controlled by two genes - the colour epistatic gene (W/w) and the yellow gene (Y/y). The no-colour, white allele (W) is dominant over the coloured allele (w), so WW or Ww will be white and ww will be coloured. The yellow gene has the dominant yellow allele (Y) and the recessive green allele (y)

Answer 68

Question 69

What is the chi squared test

Answer 69

The chi-squared test (χ2) is a statistical test that’s used to see if the results of an experiment support a theory

Question 70

How does the chi squared test work

Answer 70

The theory is used to predict a result
This is called the expected result.
Then, the experiment is carried out and the actual result is recorded
This is called the observed result.
To see if the results support the theory you have to make a null hypothesis
The χ2 test is carried out and the outcome either support or rejects null hypothesis

Question 71

What is a null hypothesis

Answer 71

The null hypothesis is always that there’s no significant difference between the observed and expected results

Experimental result will usually be slightly different from what you expect
You need to know if difference is due to chance, or because the theory is wrong

Question 72

Where can the χ2 be used

Answer 72

Can be used to test theories about the inheritance of characteristics

Question 73

Chi squared test equation

Answer 73

O: observed results
E: expected results

Question 74

Steps to work out chi squared test

Answer 74

Make table of phenotypes and the expected ratio based off of what type of inheritance they are
(E.g. dihybrid ratio is expected to be 9:3:3:1)
Then put in expected results of each phenotype (E)
(E.g. with dihybrid, out of 160 there is 90,30,30 and 10 fitting into ratio
Fill in observed results (O)
Carry out steps of equation: O-E, (O-E)^2, (O-E)^2/E and add them together to get χ2 (calculated value)
Put calculated value into critical value table to test significance

Question 75

How to find out if calculated value is significant using critical value table

Answer 75

If your χ2 value is larger than or equal to the critical value then there is a significant difference between the observed and expected results (something other than chance is causing the difference)
The null hypothesis can be rejected.
REJECT TO THE RIGHT
If calculated value is smaller than critical value there is no significant difference between observed and expected results so null hypothesis can’t be rejected

Question 76

How to use a critical value table

Answer 76

E.g. 0.05 means there is a 5% probability that difference between expected and observed value is due to chance

First work out degrees of freedom
This is number of classes - 1
Then find critical value at the specific degree of freedom and compare calculated value to it