a sequence of bases on a DNA molecule that codes for a protein, which results in a charatceristic
alternative versions of the same gene
2 alleles of each gene = one from each parent
different versions of the same characteristic eg. eye colour, blue eyes or brown eyes etc
the genetic constitution of an organism
the expression of the genetic constitution of an organism and its interaction with the enviroment
ie what version of the allele is actually expressed, eg just blue eyes
define dominant alleles
an allele whose characteristic appears in the phenotype , even when theres only one copy
Show by a capital later, ie BB, Bb
define recessive alleles
an allele whose characteristic only appears in the phenotype if 2 copies are present
alleles that are both expressed in the phenotype
neither is recessive
eg. sickle cell anemia , heterozygous for sickle cell have a sickle cell trait
fixed position of a gene on a chromosome
alleles of a gene are found at the same locus on each chromosome
an organism that carries 2 copies of the same allele
eg BB, bb
an organism that carries 2 different alleles
what is monohybrid inheritance?
individuals have 2 alleles for each gene
Gametes contain only one allele for each gene
when gametes fuse, the alleles they contain form the genotype of the offspring
monohybrid inheritance is the inheritance of a single characteristic (gene) controlled by different alleles.
monohybrid crosses show the likelihood of alleles being inherited by offspring of particualr parents
punnet squares show the same thing as monohybrid crosses , a different style of genetic diagram
explain sex linkage
females have XX chromosomes, males have XY
probability of having male/female offspring is always 50/50
sex linked, - when the allele that codes for a characteristic is located on the sex chromosome
the Y chromosome is smaller than the X chromosome - so carries fewer genes
most sex linked genes are X-linked
as males only have one x-chromosome, they only have one allele for sex-linked genes - because they only have one copy they express the characteristic even if its recessive
males more likely than females to show sex-linked phenotypes
some genes have multiple alleles
eg. blood group
caused by specific antigen- A, B, O
IO = recessive
what is a gene pool and allele freq.
the complete range of alleles present in a popualtion
how often an allele occurs is allele frequency
what are the assumptions of HW?
-mating must be random
equation for predicting allele freq
p= frequency of dominant allele
q= frequency of recessive allele
equation for predicting genotypye freq.
p2= frequency of homozygous dominant
2PQ= frequency of heterozygous
q2= frequency of homozygous recessive
what is differential reproductive success?
it affects allele frequency in a population
when the allele that codes for a characteristic affects the chances of an organism surviving
not all individuals are as likely to reproduce as each other
its a part of natural selection as individuals that have the allele that increases their chance of survival are more likely to survive and reproduce , so the frequency of the beneficial allele increases from generation to generation
what is stabilising selection?
individuals towards the middle of the range are more likely to survive and reproduce
occurs when the enviroment isnt changing, and it reduces the range of possibility phenotypes
average individuals more likely to survive and pass on allele, closest to the mean
what is directional selection?
individuals with alleles fro characteristics of an extreme type are more likely to survive and reproduce
could be in repsonse to enviromental change
not average individuals, left or right of mean
speciation is the development of a new species
-occurs when popualtions of the same species become reproductively isolated
-this can happen when a physical barrier eg flood, divides a species population =geographical isolation
-geographically seperated populations will experience different condiitions, eg climate , so different alleles will become more advantageous in the different populations, directional selection will occur
-alelle freq. will aslo change due to mutations which occur in dependantly
-the changes in allele freq. leads to differences in the gene pools of the 2 pops. = changes in phenotype freq.
-eventually the pops. have changed so much that they can no longer breed to produce fertile offspring == reproductively isolated