Inheritance - Part 2
How genetically different are any two humans from each other?
On average, they differ by about 1 in 1000 base pairs
What are the 3 main sources of genetic variation?
1. Synapsis and crossing over in prophase I
2. Independent assortment of homologous pairs at the metaphase plate
3. Seperation of homologs during anaphase I
What happens during synapsis and crossing over?
Homologous chromsomes pair up and exchange segments of genetic material.
What is independent assortment?
Its the random distribution of materal and parernal chromsomes into gametes
Hoe many possible chromosomes combinations can occur in human gametes die to independent assortment?
More than 8 million possible combinations (2^23)
What is crossing over, and how does it contribute to genetic variation?
It is the exchange of material between homologous chromosomes during Prophase I
Why is genetic variation important from an evolutionary perspective?
It provides the raw material for natural selection and evolution
What holds sister chromatids together after interphase
Proteins called cohesins
What happens to nonsister chromatids during crossing over?
They are broken at corresponding positions and then rejoined to each other
What is the synaptonemal complex?
A zipper like structure that holds homologous chromosomes tightly together during synapsis.
What is the result of DNA break repair during crossing over?
DNA from one nonsister chromatid is joined to the corresponding segment of another, forming recombinant chromosomes.
How does gene mapping work?
By tracking crossing over events to determine how far apart genes are on a chromosome
How does distance affect crossing over?
Further apart -> greater chance of crossing over
Closer togeher -> less chance of crossing over
What are linked genes?
Genes that are physically close on a chromosome and tend to be inherited together
How can one gene act as a marker for another?
If two genes are linked, detecting one can indicate the presence for another
What is random fertilisation?
The process where any sperm can fuse to any ovum
How much genetic variation results in random fertilisation in humans?
Around 70 trillion possible diploid combinations
Why does each zygote have a unique genetic identity?
Because of crossing over, independent assortmant and random fertilisation
How many humans have ever lived?
117 billion
What does natural selection result in?
The accumulation of genetic variations that are favoured by the environment
How does sexual reproduction comtribute to genetic variation?
It reshuffles alleles througj processes like independent assortment and crossing ovee, creatinf unique combinations of genes
What is the original source of genetic variation in a population?
Mutations
Are asexual animals common or rare?
Rare
How are identical twins formed?
When a zygote splits in half after fertilisation
Must identical twins be same sex?
Yes
What does the P stand for in genetics?
Parental generation - a pure breeding strain
What does F1 stand for?
First filial generation - the offspring of the parental (P) cross.
What does F2 stand for?
Second filial generation, produced by self-fertilising the F1 plants
What was Mendel's first concept?
Alternative versions of genes account for variations in inherited traits
What are these alternative versions of a gene called?
Alleles
Where does each gene reside?
At a specifc locus (postition) on a specfic chromosome
What was mendel's second concept?
Each organism inherits two alleles for each trait - one from each parent
What is a genotype?
An organisms set of genes or alleles
What is a phenotype?
The physical or observable expression of the genotype
What happens when the two alleles at a locus differ?
The dominant allele determines the organisms appearence, while the recessive allele has no visible effect
How did this appear in Mendels flower-colour experiment?
The F1 plants had purple flowers because the purple allele was dominant and the white allele was recessive
How is genetic information inherited?
As discrete units or factors, now knowm as genes
Hoe many alleles does a diploid organism have for each gene?
2 alleles, which may be identical or different
What is a dominant allele?
It is expressed in the phenotype when present
What is a recessive allele?
It is only expressed when both alleles are recessive
What happens to alleles during gamete formation?
The two alleles segregate, so each gamete receives only one per gene
What is gene linkage?
When genes close together on the same chromosome tend to be inherited together.
What was gene linkage originally used for?
To determine the locations of genes on chromosomes
Do all human traits follow Mendelian patterns?
Most do especially single gene disorders but not all
Why are humans not ideal subjects for genetic research?
Long generation times
Ethical issues
Small family sizes
Why is the relationship between genotype and phenotype not always clear?
Because some genes have multiple alleles or interact with other genes and environmental factors
What is complete dominance?
When the heterozygote and the dominant homozygote have identical phenotypes
What is incomplete dominance
When the heterozygotes phenotype is intermediate betweem the two parental phenotypes
What is codominance?
When both alleles are expressed equally and affect the phenotype in distinct ways