genes, alleles and inheritance Flashcards
what is a gene
a section of DNA that determines the sequence of amino acids in the protien
what is a genotype
the genetic constitution of an organism ( all the genes it has )
what is a phenotype
the expression of a genotype, alleles and interaction with the environment ( physical appearance )
what is an allele
diferent forms of a gene
what are homologous chromosomes
chromosomes which have the same genes at the same loci but not necessarily the same alleles
what is a locus
different alleles for the same gene are found at the same position on homologous chromosome, this position is referred to the locus of that gene
what is meant by homozygous
two copies of a gene are the same allele
what is meant by heterozygous
two copies of a gene are different alleles
what is meant by dominant alleles
always expressed in the phenotype if the genotype is homozygous
what is meant by co dominant alleles
both alleles are expressed in the phenotype if the genotype is homozygous
what is meant by diploid
contains two alleles of every gene e.g. adult cells
what is meant by haploid
contains one allele of each gene e.g. gametes ( sperm )
why are fruit flies used to study genetic crosses
- large number of offspring produced which improves reliability by reducing sampling errors and allows statistical tests to be done
- small size so can breed in small flasks on simple nutrient mediums which reduces costs and means they are easily stored
- distinct phenotypes are easily observed and gender is easy to identify
- short generation time means they reproduce quickly so you can obtain many generations in a short time
what is monohybrid inheritance
the inheritance of a single gene which determines a single characteristic .
e.g. phenotype is determined by 2 alleles one of which is dominant and the other is recessive
why are actual rations different from expected ratios
- fertilisation of gametes is random
- random mutations
- gene linkage
what is co dominance
both alleles contribute equally to the phenotype
when do multiple alleles exist
where there is more than two alleles at the same locus
what is sex linkage
a sex linked character is controlled by a gene located on one of the sex chromosomes. A sex linked gene is on the X chromosome.
the y chromosome only carries sex linked genes
the x chromosomes carries both sex genes and genes controlling other body characteristics
what is dihybrid inheritance mean
involves a phenotype that is inherited as a result of two different genes. Involves the same principles you learned carrying out monohybrid crosses, however the number of possible phenotypes increases
what is the phonotypic ratio in a dihybrid cross when one parent is heterozygous for both genes ( TtAa) and one parent is homozygous recessive ( ttaa)
1:1:1:1
what is the phenotypic ratio in a dihybrid cross when both parent are heterozygous for both genes ( TtAa)
9:3:3:1
