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
Show the genetic cross for two adults which are heterozygous with sickle cell trait (codominance)
What are the alleles for ABO blood type
Show genetic cross between person with heterozygous blood type A and heterozygous blood type B
Work out inheritance for heterozygous pea plant parents with round and yellow seeds
Table of F1 and F2 phenotype ratio with differing homozygous parents for monogenic, dihybrid and codominance
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
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.
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.
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
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)
Chi squared test equation
O: observed results
E: expected results
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
