Patterns of inheritance and variation Flashcards
What is phenotypic variation?
Variation shown in the characteristic of an organism (phenotype)
Give an example of phenotypic variation
human blood group
What is polygenic variation?
Variation influenced by many genes
Give an example of polygenic variation
Human skin colour
What is monogenic variation?
Variation influenced by only one or a small number of genes
Give an example of monogenic variation
Violet flowers, either coloured or white
How can variation be categorised?
continuous and discontinuous
What are alleles?
different versions of genes within the same species
What is etiolation?
plants don’t get enough light so grow spindly and long
What is chlorosis?
plants don’t get enough magnesium so don’t produce enough chlorophyll and turn yellow
What two factors can affect variation?
genetic and environmental
Give two examples where genetic and environmental factors affect phenotype variation
- body mass in humans
- dwarf and tall pea plants
How is genetic variation achieved during meiosis?
- crossing over of non-sister chromatids (prophase 1)
- independent assortment of homologous chromosomes (metaphase 1)
- independent assortment of chromatids (metaphase 2)
- independent segregation of chromatids (anaphase 2)
What is monogenic inheritance?
the inheritance of a single gene
Is cystic fibrosis caused by a dominant or a recessive allele?
recessive
What did Gregor Mendel do?
Mendel experimented genetic crosses using pea plants. He used true breeding plants to breed together and studied the outcome.
What are true breeding plants?
plants that were homozygous for traits
What were the offspring of true breeding crosses called?
F1 generation
In homozygous crosses, what are the offspring?
They are all heterozygous
What is a heterozygous cross?
where 2 of the F1 offspring are taken and crossed
What are the offspring of heterozygous crosses called?
F2 generation
What is the result of a heterozygous cross?
3 offspring with dominant trait to 1 offspring with the recessive characteristic
What is codominance?
when 2 different alleles are equally dominant so both alleles are expressed in the phenotype
Give and describe an example of codominance
Snapdragons:
Allele for red flowers: codes for enzyme that catalyses the production of red pigment from a colourless precursor
Allele for white flowers: codes for an altered enzyme that doesn’t code for a pigment
Give an example of a trait that has more than two different alleles
blood groups
What are blood groups determined by?
the immunoglobulin gene (gene I) coding for antigens present on the surface of blood cells
What does I A produce?
antigen A (spherical shape)
What does I B produce?
antigen B (triangle shape)
What does I O produce?
no antigen
Which two alleles for blood groups are codominant?
I A and I B
Which allele for blood groups is recessive to the other two alleles?
I O
What two genotypes express blood group A?
I A I A and I A I O
What two genotypes express blood group B?
I B I B and I B I O
What genotype expresses blood group AB?
I A I B
What genotype expresses blood group O?
I O I O
How many chromosomes are there in a human body cell?
46
What are the two chromosomes found in males?
XY
What are the two chromosomes found in females?
XX
Give an example of a characteristic determined by a gene linked to X and Y chromosomes
Haemophilia
What is haemophilia?
slow blood clotting due to absence of clotting factor
What does XH represent?
the dominant ‘healthy’ allele on the X chromosome
What does Xh represent?
the recessive allele coding for haemophilia on the X chromosome
What does Y represent?
the Y chromosome which has no allele attached to it
What is dihybrid inheritance?
the inheritance of 2 genes
What is the expected phenotypic ratio of the F2 generation of a dihybrid cross test?
9:3:3:1
Why might the phenotypic dihybrid cross-ratio differ in reality?
- fertilisation is random
- sometimes the genes may be linked so always inherited together
What is autosomal linkage?
It is the inheritance of linked genes on the chromosomes other than those that determine sex (XX & XY)
How are linked genes inherited together?
They don’t usually undergo independent assortment but they may sometimes be separated by crossing over.
If alleles are linked, what does this produce?
more gametes containing that combination of linked alleles
What are recombinant allele combinations?
Allele combinations that are only produced occasionally during crossing over
What is x^2/ chi-squared for?
statistical test to measure if there is a significant difference between the observed and expected results in a genetic cross
What is the null hypothesis of genetic crosses?
There is no significant difference between the observed and expected and any difference is due to chance alone.
What is the equation for chi-squared?
x^2 = the sum of (O - E)^2 / E
How do you work out the critical value?
Usually use 0.05 and the degrees of freedom is the number of possible outcomes - 1
Do we accept or reject the null hypothesis when x^2 is less than the critical value?
accept and we say there is no significant difference between the observed and expected, any difference must be due to chance
Do we accept or reject the null hypothesis when x^2 is more than the critical value?
reject and we say that there is a statistically significant difference between the observed and expected so there must be a reason for this
Which type of graph can be used to present continuous data?
line graph
Which type of graph can be used to present discontinuous data?
bar chart
