18. Genetics and Inheritance Flashcards
Genotype
The genetic consitution of an organism
Phenotype
The expression of a genotype and its interaction with the environment
What is a gene
A sequence of bases on DNA that codes for one polypeptide
What are alleles
Alternative forms of a gene
What is a locus
The position of a gene on a chromosomes
What is a dominant allele
Always expressed in the phenotype if its presentin the genotype
Recessive allele
Only expressed in the phenotype if the dominant allele is absent
Why is it important for an organism to have some characteristics that are catergoric when invesigating inheritence
So can clearly distinguish the characteristics tested - no intermediates
Characteristics will be genetically determined
Why must an oranism reproduce sexually when investigating inheritence
So can see the effect or recombining alleles and get variation in the offspring
Why is it important of an organisms to produce large numbers of offspring when investigating inheritence
-Will give representative data
-Large enough sample size
-Enough data for statistical analysis
Why is it important for an organisms to have a short lifespan when investigating inheritance
Can obtain data quickly
Can observe several generations
Why is it important for an organisms to be small when investigating inheritance
Easily kept
Reduces costs
When is a monohybrid cross used
If inheritance of one gene coding for one characteristic, is studied
Whats a testcross/ backcross
To determine which genotype an organism has by crossing it with an organism with a homozygous recessive phenotype
If any offspring are homozygous recessive then the unknown genotype is heterozygous
If an offpring, from an unknown organisms during backcrossing, is dominant, why is the unkown genotype is only tentative
-random fertilisation of gametes, could just be heterozygous offspring by chance
-Large number of offspring
What are codominant alleles
Those that are both expressed in the phenotype of a heterozygous organism
Gives rise to a third phenotype
What is sex determination
Sex is determined by the combination of chromosomes in a cell / organism rather than the genes themselves
What is sex linkage
Genes are carried on the sex chromosome and are inhertited in different ways in males and fremales
What are the sdex chromosomes in males and femles
Males =X Y , Non-homologous
Female n = X X
Why are males more likley to have the reccessive versions of the phenotype
Only one recessive allele needs to be present in order for it to be expressed
Women need 2
What is the evidence for alleles being dominant / recessive in pedigree diagrams
-Look for where an offspring has a different phenotype to the parents
What is evidence for sex-linkage in pedigree diagrams
-Look for much greater numbers of affected males than females, or no affected females at all
What is evidence against sex-linkage in pedigree diagrams
-Look for where a female with recessive phenotypes has a son or father with the dominant phenotype
What is a dihydbrid cross
Consideres have 2 different genes are inherited
Organisms will have 4 alleles in their genotype and the gametes produced by each individual will jhave 2 alleles
What is epistasis
arises in a dihybrid cross when there is some form of interaction between the 2 genes, One is said to be epistatic over the other
What is the common dihybrid ratio
9;3;3;1
When are dashes used in genotypes
To show that either allele of a gene could give the same phenotype
What is autosomal linkage
All the non-sex chromosomes
They vary in the number of genes they carry
2 genes on the same chromosome
Why are genes on the same chromosomes said to be linked
they will stay together during independent assortment of chromosomes in the first division of meiosis.
their alleles will be passed on to the offspring together so fewer types of gametes will form. In a dihybrid cross this can result in unexpected offspring ratios.
How likley is crossing over to occur
The closer together the genes are on the chromosomes, the less likely it is that crossing over will result in these recombinant genotypes.
A smaller proportion of recombinant genotypes indicates that the genes are closer together.
