Monogenetic + Dihybrid Inheritance 20.2/3 Flashcards
How do you show how genes are passed on?
You can use genetic crosses
Monogenetic inheritance
Showing how a single gene has been inherited.
How do you perform a genetic cross? List the 6 steps.
1) State phenotype of parents
2) State genotype of parents using capital for dominant and lower case for recessive
3) State the gametes of each parent as a G
4) Use a punnet square to show fertilisation results
5) State the proportion of each genotype
6) State what phenotype this corresponds to.
Homozygous genetic cross
Mendel carried out a genetic cross on a homozygous green pea pod and a homozygous yellow pea pod. The green pea pod was dominant and all of the offspring were heterozygous so ended up being green.
Heterozygous genetic cross
This is where two heterozygous green pods were genetically crossed.
What type of generation is the offspring?
The F1 generation
Give an example of codominance
An allele that codes for a red flower will code for an enzyme that produces red pigment. An allele that codes for a white flower will code for a different version of the enzyme that won’t produce the pigment. If both these alleles are present a pink flower will be formed.
What notation do you use for codominant alleles?
You can’t use capital and lower case as they are both dominant so you use the same letter with a different superscript e.g C(w) and C(r)
What happens if you have genes with multiple alleles?
Some genes will have more than 2 alleles for each gene however an organism can only carry two alleles because they one on each of the homologous chromosomes.
How do you determine sex?
Humans each have 23 pairs of chromosomes, 22 of these are homologous, the 23rd is the sex homologous chromosome. In a female, it is an xx chromosome and in a male, it is an xy chromosome. X chromosomes have many genes that don’t code for sexual development and they are very big. Y chromosomes are very small and don’t contain a lot of genetic info, however, they have a gene that codes for male development.
What are sex-linked genes?
Genes on sex chromosomes that code for certain characteristics. Sometimes a characteristic that is recessive on the x chromosome will be displayed more often in males as they only have one version of that gene whereas women will likely have a dominant allele so will not show the recessive characteristic. E.g colourblindness is more common in men because they have that recessive gene on their x chromosome and women will most likely have a dominant allele which means they are not colourblind.
What is haemophilia?
It is a sex-linked genetic disorder that causes blood to clot a lot slower than normal because they don’t have the blood clotting factor. This could cause prolonged bleeding during any injury and could be fatal.
Why can only men be haemophiliacs?
Because they only have one x chromosome and so if they inherit the recessive allele, they will get it as they don’t have any dominant allele on their y chromosome as it doesn’t carry genetic information. Women will only be carriers if they have heterozygous chromosomes for the haemophilia gene.
What is dihybrid inheritance?
Its the inheritance of two genes with two different characteristics instead of one for example a seed that is green and wrinkled.
What is the offspring of a genetic cross called
F1 offspring
What is selfing
When you take two F1 offspring organisms and genetically cross them
What is the offspring of selfing called?
F2 offspring
What happens when you do a dihybrid cross for two homozygous organisms for both genes?
You get a 2x2 square table with 100% heterozygous
What happens when you do a dihybrid cross for two heterozygous organisms for both genes?
You get a 4x4 square with a 9:3:3:1 ratio
Why might the ratios of genetic crosses be different than expected?
- Fertilisation is random so a few chance events can be different
- The genes could be on the same chromosome so crossing over wouldn’t happen and they would be inherited together.
Why might the ratios of genetic crosses be different than expected?
- Fertilisation is random so if the sample size is small a few chance events could skew the ratios
- The genes could be on the same chromosome and linked so crossing over wouldn’t happen and they would be inherited together.
What is codominance
When two alleles occurring in the same gene are equally dominant and will both be expressed in the phenotype of a heterozygous organism.
