Patterns of inheritance Flashcards
Continuous variation
when the individuals in a population vary within a range- there are no distinct categories
e.g. height, weight
Discontinuous variation
when there are 2 or more distinct categories where an individual falls into 1 only- there are no intermediates
e.g. blood types, tongue-rolling ability
Phenotypic variation
variation in an organism’s phenotypes. this is influenced by different factors like genotypes, environment, or both combined.
What makes up an organism’s genotype?
their genes and alleles
Individuals of the same species have the same genes but different?
alleles
How can differences in genotypes occur?
1- sexual reproduction leads to variation in genotypes within a species
2- meiosis makes gametes with unique assortment of alleles through crossing-over and independent assortment of chromosomes
3- a random fusion of gametes during fertilisation
What are inherited characteristics that show continuous variation usually influenced by?
many genes
What are inherited characteristics controlled by many genes called?
Polygenic
e.g. human skin colour
What are inherited characteristics that show discontinuous variation usually influenced by?
only one gene or a small no. of genes.
What are characteristics controlled by only one gene called?
Monogenic
e.g. violet flower colour (either coloured or white)
Give examples of the environment causing phenotypic variation.
1- Etiolation: plants grow abnormally long and spindly due to lack of light
2- Chlorosis: plants don’t produce enough chlorophyll and turn yellow due to factors like lack of magnesium in the soil.
What type of variation does phenotypic variation controlled by both G+E show?
continuous variation
Give examples of both genotype and the environment causing phenotypic variation.
1- Body mass in animals
2- Height of pea plants- they come tall or dwarf (genetic) but it can vary due to factors like light intensity and water availability.
What is a gene?
a sequence of bases on a DNA molecule that codes for a protein which results in a characteristic
What is an allele?
different version of a gene
Why do we have 2 alleles of each gene?
We inherit one copy of each chromosome of a pair from our parents.
What is a locus?
A fixed position where the allele of each gene is found.
What are codominant alleles?
alleles that are both expressed as neither are recessive
What do genetic diagrams show?
the possible genotypes of offspring
What is the expected ratio for monogenic crosses?
3:1
What is dihybrid inheritance?
inheritance of 2 characteristics which are controlled by diff. genes. each of the 2 genes have different alleles.
What is the expected ratio for a dihybrid cross of 2 heterozygous parents?
9:3:3:1
What does it mean if a characteristic is sex-linked?
the allele it codes for is located on a sex chromosome
Why are most genes on the sex chromosomes found on the X chromosome?
the Y chromosome is smaller and carries fewer genes
Why are males more likely to express sex-linked genes even if its recessive?
they only have one allele for sex-linked genes.
What are 2 examples of genetic disorders caused by faulty alleles on the sex chromosomes?
colour blindness and haemophilia
What do we call males since they have 2 diff. kinds of sex chromosomes?
heterogametic
What do we call females since they have one kind of sex chromosome?
homogametic
Why is colour blindness rarer in women?
they need 2 copies of the recessive allele, while males need one.
What is an autosome?
a chromosome that isn’t a sex chromosome
Why are genes on the same autosome said to be linked?
they stay together during the independent assortment of chromosomes in meiosis 1 and their alleles are passed on to the offspring together.
What problem could arise with linked autosomal genes?
crossing over could split them up first.
The closer together 2 genes are on the autosome…
the more closely they are linked as crossing over is less likely to split them up
What is the expected ratio of 2 autosomally linked genes in a dihybrid cross?
it would be expected to be 9:3:3:1 but is actually like a monogenic cross between 2 hetero parents, 3:1 as the genes are inherited together. a higher proportion of the offspring will have their parents’ genotype and phenotype.
What is epistasis?
when the allele of one gene blocks the expression of the alleles of other genes. they interact to form the phenotype.
Epistasis- widow’s peak example
a widow’s peak is controlled by one gene and baldness is controlled by others. if you have the alleles that code for baldness, the one for the widow’s peak isn’t expressed as you have no hair. the baldness genes are epistatic to the widow’s peak gene as it masks its expression.
Epistasis- flower colour example
it is controlled by 2 genes. gene 1 codes for a yellow pigment (Y is the dom. yellow allele) and gene 2 codes for an enzyme that turns the yellow pigment orange (R is the dom. orange allele). if you don’t have the Y allele, having the R allele doesn’t matter as the flower will be colourless. gene1 is epistatic to gene 2.
What does the phenotypic ratio from a dihybrid cross with an epistatic allele depend on?
it depends on whether the epistatic allele is recessive or dominant
What does it take for a recessive epistatic allele to make the expression of other genes?
2 of them
