Molecular Biology & Genetics 5 - Mendel's Genetics, Extended + X-Linked Traits + Recombination Flashcards
What did people think prior to Mendel’s discoveries?
When two different traits reproduced together they would just blend
What did Mendel find?
Traits didn’t blend and that some traits are dominant when crossed with different alleles of the same trait
How is dominance shown?
When two individuals pure breeding for the dominant and recessive traits are crossed all of the offspring exhibit the dominant trait
What is F1?
1st Filial generation
What happens when two of the F1 offspring are crossed?
The recessive trait may appear again in a 3:1 ratio
What is a monohybrid cross?
When one trait is being examined in the cross
What was the first of Mendel’s Conclusions?
Information passed on from parents to offspring if information is packaged as discrete particles. Mendel called these factors. We now call them genes
What was the second of Mendel’s Conclusions?
It also appears that individuals have 2 copies of each gene, one inherited from the mother and the other from the father. Genes controlling a trait can differ- these different forms of a gene are called alleles
What happens in a pungent square?
The female and male gametes are crossed to show the potential offspring
What is the genotypic ratio of a cross of 2 heterozygous individuals?
2 homozygous (1 dominant & 1 recessive) and 2 heterozygous
What is the phenotypic ratio of a cross of 2 heterozygous individuals?
3:1 because the difference between homozygous dominant and heterozygous cab’t be determined
What is the aim of a test cross?
To find out whether an individual with the dominant trait is homozygous dominant or heterozygous
How is a test cross done?
By crossing the unknown individual with a homozygous recessive individual and observing the results
What happens when the unknown individual in a test cross is homozygous dominant?
All offspring show the dominant phenotype so the plant must be homozygous. However, a large amount of repeats must be done to ensure this
What happens when the unknown individual in a test cross is heterozygous?
Half of the offspring have the dominant trait and half have the recessive so the plant must be heterozygous
What is Mendel’s 1st Law?
The law of segregation
What is Mendel’s 2nd Law?
The law of independent assortment
What are Mendel’s laws explained by?
Chromosomal behaviour during meiosis
What is the law of segregation?
Genes segregate at meiosis so that each games contains only one of the two possessed by the parent
What is the law of independent assortment?
Alleles of different genes assort independently during gamete formation
Genes on chromosome can align…
in many different ways
Separation of genes is not…
influenced by the movement of any other genes
What is a dihybrid cross?
When two traits are being examined in the cross
What ration is given by a dihybrid cross?
A 9:3:3:1 ratio
What is the product rule?
Independent events are multiplied to get the probability of them all occurring
What is the sum rule?
Mutually exclusive events are added to get the probability of them all occurring
What is polymorphic?
When one gene has many alleles and therefore many phenotypes
How many alleles can an individual have for a polymorphic gene?
Only 2, one on each homologous chromosome
What is incomplete dominance?
When the F1 generation has an intermediate phenotype of the two parents. It seems like blending but heterozygous individuals just show incomplete dominance (dominant allele isn’t completely dominant over the recessive allele). The parental phenotypes may reappear in the F2 generation
What are recessive alleles?
Like broken alleles which don’t contribute to the phenotype like the dominant alleles do
What is co-dominance?
Where both phenotypes exist side by side within the organism (are equally as dominant as each other)
Can co-dominant alleles still have a recessive allele?
Yes, alleles can be co-dominant to each other and both dominant over the recessive allele
What are polygenic traits?
Traits of which the phenotype is controlled by many genes which have an additive effect.
How do polygenic trait characters appear?
Continuous or quantitative
What distribution is typical of a polygenic trait?
Normal distribution
What happens when more genes are involved in a polygenic trait?
There are more phenotypic classes
What also effects phenotype?
The environment
What does the environment do?
Smooth differences between phenotypes. Although the genetics doesn’t change, the phenotype does. The greater the effect on the environment the less the grouping/categories of phenotypes
What do sex-linked traits involved?
Genes on the sex chromosomes (X or Y) rather than autosomes
What happens with the inheritance patterns of sex-linked traits?
They differ between males (XY) and females (XX)
What does hemizygous mean?
Only a single copy of a gene in a diploid organism (e.g. males)
What does it mean if a gene is linked to the X chromosome in a male?
They will inherit the trait as they only have one x chromosome
How many genes are there in humans?
Around 20,000
How many chromosomes are there in humans?
23 pairs
How many linked genes per chromosome?
Around 1000
How are linked genes inherited?
Linked genes show biases towards the parental genotypes as they don’t recombine
How can we determine the order and location of genes within a chromosome?
By using the recombination frequencies
What is the ratio expected when a heterozygote and a homozygote recessive cross in a test cross?
1:1:1:1
Is a 1:1:1:1 ratio seen in a test cross with a heterozygote and a homozygote recessive?
No, not always. When the genes are linked most offspring will be the parental types with a few recombinants
What should theoretically be given by meiosis?
All gametes having parental genotypes
What do we get from meiosis?
Parental genotypes and some recombinants
What explains recombination?
Crossing over
What is recombinant frequency?
The proportion of recombinant genes
What does distance have to do with crossing over?
The smaller the distance between two genes the less likely it is that a chiasma will form between them
What has recombination frequencies around 50%
Distant (unlinked) genes which means they have around half recombinant and half parental genotypes
What has recombinant frequencies of 0-50%?
Close (linked) genes
What is the relationship between distance and recombination frequency?
Linear
What can recombination frequency be used to work out?
How far away genes are on a chromosome in centimorgan (cM)
What is the formula for working out how far away genes are on a chromosome?
cM = recombination frequency x 100
What are the daughters from a cross with a female dominant homozygote (AA) and a father with trait (aY)?
All Carriers
What are the sons from a cross with a female dominant homozygote (AA) and a father with trait (aY)?
All Normal
What are the daughters from a cross with a female recessive homozygote (aa) and a normal male (AY)?
All Carriers
What are the sons from a cross with a female recessive homozygote (aa) and a normal male (AY)?
All affected
What are the sons from a cross with a female carrier (Aa) and a normal male (AY)?
Half normal, half affected
What are the daughters from a cross with a female carrier (Aa) and a normal male (AY)?
Half normal (homozygous dominant), half carriers
What are the sons from a cross with a carrier female (Aa) and a male with the trait (aY)?
Half normal, half affected
What are the daughters from a cross with a carrier female (Aa) and a male with the trait (aY)?
Half carriers, half with trait
