Module 6 Section 2: Patterns of Inheritance Flashcards
What is continuous variation
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
Example of continuous variation
Height
Waist circumference
Fur length
What is discontinuous variation
When there are two or more distinct categories
Each individual falls into only one of these categories
There are no intermediates
Example of discontinuous variation
Blood group (ABO groups or Rhesus + or - grouping)
Violets (flowers) can either be coloured or white
How is variation influenced by genes
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
How do genotypes vary via sexual reproduction
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
What do differences in genotypes create (with example)
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
How many genes are inherited characteristics that show continuous variation usually influenced by
Continuous variation influenced by many genes
These are polygenic
E.g. human skin colour is polygenic - comes from lots of different shades of colour
How many genes are inherited characteristics that show discontinuous variation usually influenced by
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
How can variation be influenced by the environment
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
Examples of how variation be influenced by the environment
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
How can variation be influenced by genetics and the environment
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.
Examples of how variation can be influenced by genetics and the environment
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.
Gene
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
Allele
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).
Genotype
The alleles an organism has, e.g. BB, Bb or bb for eye colour.
Phenotype
The characteristics the alleles produce, eg. brown eyes.
Dominant
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
Recessive
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.
Codominant
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
Locus
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
Homozygote
An organism that carries two copies of the same allele, e.g. BB or bb
Heterozygote
An organism that carries two different alleles, e.g. Bb.
Carrier
A person carrying an allele which is not expressed in the phenotype but that can be passed on to offspring
How is the genotype for the offspring produced
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
What can genetic diagrams be used for
Genetic diagrams can be used to predict the genotypes and phenotypes of the offspring produced if two parence are crossed (bred)
How to show dominant and recessive alleles in genetic diagrams
Dominant: CAPITAL LETTER
Recessive: lower case letter
How to set out punnet square correctly
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
What are the characteristics of monogenic inheritance
Single gene locus, multiple alleles
Patterns of dominance, recessiveness, codominance or multiple genes acting epistatically
What are the characteristics of polygenic inheritance
Multiple genes influencing a particular characteristic
Multiple genes with multiple alleles
Tend to show normal distribution
What is incomplete dominance
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
What is monohydbrid inheritance
Inheritance patterns for a single gene locus (location on a gene on the chromosome)
What would you called these genotypes
GGYY
Ggyy
GGYY: homozygous dominant at both gene loci
Ggyy: Heterozygous dominant at G locus, homozygous recessive at Y locus
How is sickle cell anaemia show codominance
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
Show the genetic cross for two adults which are heterozygous with sickle cell trait (codominance)
What are the alleles for ABO blood type
How does ABO blood grouping show codominance
I^O is recessive
I^A and I^B are codominant
People with genotype I^A I^B will have blood group AB
Show genetic cross between person with heterozygous blood type A and heterozygous blood type B
What is dihybrid inheritance
The inheritance of two characteristics which are controlled by two different genes
Worked out using a dihybrid cross
Work out inheritance for heterozygous pea plant parents with round and yellow seeds
What is the phenotypic ratio
The ratio of different phenotypes in offspring
F1 and F2 predicted using genetic diagrams
Table of F1 and F2 phenotype ratio with differing homozygous parents for monogenic, dihybrid and codominance
What can change the phenotypic ratio from the normal
Can be because of sex linkage, autosomal linkage or epistasis
What are the genotypes of the different sexes
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)
What is sex linkage
A characteristic is said to be sex-linked when the allele that codes for it is located on a sex chromosome
Why are males more likely to get genetic disorders
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
Examples of sex linked disorders
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
How is colourblindness a sex linked disorder
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.
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
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
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)
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
What is an autosome and what is an autosomal gene
Autosome is the name for any chromosome that isn’t a sex chromosome
Autosomal genes are the genes located on the autosomes
Why are genes on the same autosome linked
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.
When does independent assortment and crossing over happen
Crossing over (prophase 1)
Independent assortment of chromosomes at metaphase 1
Independent assortment of sister chromatids at metaphase 2
What happens to the phenotypic ratio if the two genes are autosomally linked
The phenotypic ratio won’t be what is expected
What may happen to the F2 ratio on a dihybrid cross if the genes turn out to be autosomally linked
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
What is epistasis
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
Where on the genes does epistasis refer to
Epistasis describes an interaction between genes at different loci
Affecting a single phenotypic trait
What is recessive epistasis
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
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
You will get a 9: 3: 4 phenotypic ratio of dominant both: dominant epistatic recessive at other locus : recessive epistatic IN THE F2 GENERATION
How is baldness and widows peak an example of epistasis in humans
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
How is flower pigment an example of epistasis in plants
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.
Are crosses of epistatic genes the same as normal crosses
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
Mechanisms behind how recessive epistasis results in differing phenotypes in pigmented plants
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
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
What is dominant epistasis
A dominant allele at the epistatic gene ‘masks’ the hypostatic gene
(even if there is a dominant allele present at the hypostatic gene locus)
Phenotypic ratio of F2 generation when crossing a homozygous recessive parent with a homozygous dominant parent for dominant epistasis for dihybrid inheritance
Will produce a 12: 3: 1 phenotypic ratio for dihybrid inheritance of dominant epistatic: recessive epistatic dominant other : recessive both in the F2 GENERATION
Mechanisms behind how dominant epistasis results in differing phenotypes
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
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)
What is the chi squared test
The chi-squared test (χ2) is a statistical test that’s used to see if the results of an experiment support a theory
How does the chi squared test work
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
What is a null hypothesis
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
Where can the χ2 be used
Can be used to test theories about the inheritance of characteristics
Chi squared test equation
O: observed results
E: expected results
Steps to work out chi squared test
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
How to find out if calculated value is significant using critical value table
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
How to use a critical value table
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
