7 Genetics Flashcards
genotype
genetic make up of an organism
describes all the alleles that an organism has
phenotype
the observable or biochemical characteristics of an organism.
it is the result of the interaction between the the expression of the genotype and the environment
gene
a length of dna that normally code for a particular polypeptide
genes exist in two or more different forms called alleles
the position of a gene on a particular dna molecule is known as the locus
allele
one of the different forms of a gene
only one allele of a gene can occur at the locus of any one chromosome
Diploid organisms have two alleles (one on each chromosome).
There may be many alleles of a single gene.
Alleles may be dominant, recessive or codominant.
In a diploid organism, the alleles at a specific locus may be either homozygous or heterozygous.
homozygous
if the allele on each of the homologous chromosomes is the same
heterozygous
if the two alleles on the homologous chromosomes are different
homologous chromosomes
in diploid organisms the chromosomes occur in pairs called homologous chromosomes
there are therefore two loci that each carry one allele of a gene
dominant
the allele of the heterozygote that expresses itself is said to be dominant
recessive
the allele of the heterozygote that doesn’t express itself is said to be recessive
homozygous dominant
a homozygous organism with two dominant alleles
homozygous recessive
a homozygous organism with two recessive alleles
diploid organism
organism with 1 set of homologous chromosome, i.e., organism has two copies of same gene in the system. humans for example has 23 pairs of homologous chromosomes, with 2 copies of each gene on two chromosomes
codominant
when two alleles both contribute to the phenotype
the phenotype is either a blend of both features or both features are presented
both alleles are equally dominant
monohybrid inheritance
the inheritance of a single gene
e.g. gregor mendel and colour of pea plants
example of monohybrid inheritance- inheritance of pod colour in peas
if pure-breeding green-pod plants are crossed with pure-breeding yellow-pod plants, all the offspring, known as the first filial, or F1, generation, produce green pods. this means that the allele for green pods is dominant to the allele for yellow pods, which is therefore recessive
when the heterozygous plants (Gg) of the F1 generation are crossed with another (=F1 intercross), the offspring (F2 generation) are always in an approximate ratio of three plants with green pods to one plant with yellow pods
these observed facts led to the formation of a basic law of genetics- the law of segregation
law of segregation
states that in diploid organisms, characteristics are determined by alleles that occur in pairs. only one of each pair of alleles can be present in a single gamete
dihybrid inheritance
a cross between two different genes that differ in two observed traits.
e.g. gregor mendel and pea seed shape and colour
example of dihybrid inheritance- inheritance of pea seed shape and colour
- round shape dominant to wrinkled shape
- yellow colour dominant to green colour
he carried out a cross between two pure breeding types of plants:
- one always producing round shaped, yellow seeds
- one always wrinkled and green
in the F1 generation he obtained plants all of which produced round yellow seeds, dominant features.
he raised the plants from these seeds and crossed them
produced 4 types of gamete (RG Rg rG rg) as gene for seed colour and gene for seed shape are on separate chromosomes
ratio of 9:3:3:1
observations led him to formulate his law of independent assortment
law of independent assortment
each member of a pair of alleles may combine randomly with either of another pair
multiple alleles
where there are more than two alleles, of which only two may be present at the loci of an individual’s homologous chromosomes
codominance example- snapdragons
one allele codes for an enzyme that catalyses the formation of a red pigment in flowers
the other allele codes for an altered enzyme that lacks this catalytic activity and so does not produce the pigment
three colours are found:
-in plants that are homozygous for the first allele, both alleles code for the enzyme, and hence pigment, production
-in plants that are homozygous for the other allele, no enzyme and hence no pigment is produced. these plants have white flowers
-heterozygous plants, with their single allele for the functional enzyme, produce just sufficient red pigment to produce pink flowers
we cannot use upper and lower case letters for the alleles
C for colour
C^R for red
C^W for white
multiple alleles example- inheritance of ABO blood groups
there are three alleles associated with the gene I (immunoglobulin gene), which lead to the presence of different antigens on the cell-surface membrane of red blood cells:
- allele I^A, which leads to the production of antigen A
- allele I^B, which leads to the production of antigen B
- allele I^O, which does not lead to the production of either antigen
only two alleles can be present in an individual at any one time, as there are only two homologous chromosomes and therefore only two gene loci
alleles I^A and I^B are codominant where as I^O is recessive to both
possible blood groups are A, B, AB or O
sex inheritance in humans
sex chromosomes are X and Y
- as females have two X chromosomes, all the gametes are the same in that they contain a single X chromosome
- as males have one X and one Y, they produce two diff types of gamete- half have an X chromosome and half have a Y
sex linkage
any gene that is carried on either the X or Y chromosome is said to be sex-linked
however, the X chromosome is much longer than the Y chromosome
this means that, for most of the length of the X chromosome, there is no equivalent homologous portion of the Y chromosome.
those characteristics that are controlled by recessive alleles on this non-homologous portion of the X chromosome will appear more frequently in the male
as there is no homologous portion on the Y chromosome that might have the dominant allele, in the presence of which the recessive allele does not express itself