genetics Flashcards
define locus
position of a gene on a chromosome
define allele
alternative form of a gene (each allele has a unique nucleotide sequence, only one of which can occur at a given locus)
what is recessive allele
allele that shows its phenotype only in the presence of another identical recessive allele.
in heterozygotes, effect of gene product expressed by recessive allele is masked by effect of gene product expressed by dominant allele
What is a dominant allele
allele that shows its phenotype regardless of whether it is homozygous or heterozygous.
in heterozygotes, gene product of dominant allele masks effect of gene product expressed by the other recessive allele.
explain codominance
when two different dominant alleles of the same gene are both expressed and influences the phenotype, resulting in a heterozygote phenotype that is different from phenotypes of both homozygotes.
explain incomplete dominance
when the phenotype of heterozygotes is intermediate between phenotypes of individuals homozygous for either allele.
what is genotype
the combination of alleles, situated on homologous chromosomes at corresponding loci. Genotype determines a specific trait of an organism and are passed to offspring
explain a homozygous condition
diploid condition in which alleles at a given locus on a pair of homologous chromosomes are identical
explain heterozygous condition
diploid condition in which alleles at a given locus on a pair of homologous chromosomes are different.
state 3 environmental factors that affect phenotypic variation
- Temperature
- UV light
- Food/nutrition
what is phenotype
characteristic (trait) of an organism that is expressed or observed.
arises from interaction of genotype and environment in which the organisms develops. Environment may alter the appearance of the organism
what is linkage
two or more genes on same chromosome that do not assort independently in meiosis and are inherited together.
genes closer together are less likely to cross over and exchange alleles between homologous chromosomes.
what is test cross
cross with a homozygous recessive individual
briefly describe the law of independent assortment
- homologous pairs of chromosomes align randomly on either side of metaphase plate during metaphase I
- alignment of each homologous pair is independent of other homologous pairs
describe monohybrid crosses
are genetic crosses that studies the inheritance of one characteristic controlled by a single gene
describe dihybrid inheritance
genetic crosses that studies the inheritance of two different characteristics controlled by two different genes
what are linked genes
are located on the same chromosome, usually located physically close enough together on a chromosome that the alleles of the two genes may be inherited together. The closer the gene loci on the same chromosome, the higher the chance that the alleles of the genes will be inherited together as one linkage group
what is sex linkage
genes that are carried on the sex chromosomes, that result in a characteristic being expressed mainly in one sex.
- sons will only obtain these features from their mothers.
- daughters get their sex-linked features from both parents.
features of discontinous variation
- there are distinct and discrete phenotypes
- generally caused by different alleles of a single gene
- their phenotypic expression are usually unaffected by environmental conditions.
features of continuous variation
- differences are slight and phenotypic differences vary along a continuum in graduations, no distinct groupings are formed
- there is a range of phenotypes for one characteristic
- usually indicates polygenic inheritance, where multiple genes are involved. There is an additive effect of each gene, where each gene has a small overall effect on a single phenotypic character.
- polygenic inheritance will result in a bell-shaped normal distribution curve, where intermediate phenotypes are more common than extreme phenotypes
- phenotypes are affect by environmental factors.
define epistasis
a form of gene interaction in which a gene at one locus alters the phenotypic expression of a gene at a second locus
there will be deviation form the 9:3:3:1 ratio in different combinations when two heterozygotes are crossed.
what is recessive epistasis
recessive allele of one gene, present in the homozygous condition, masks the phenotypic effects of alleles of the other gene
what is dominant epistasis
dominant allele of one gene, present in either homozygous or heterozygous condition, masks the phenotypic effects of alleles of the other gene.
