Unit 4; Section 5 - Inheritance, selection and speciation

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define gene

a sequence of bases on a DNA molecule that codes for a protein, which results in a charatceristic

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define allele

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

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define genotype

the genetic constitution of an organism

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define phenotype

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

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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

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define recessive alleles

an allele whose characteristic only appears in the phenotype if 2 copies are present

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define co-dominance

alleles that are both expressed in the phenotype

neither is recessive

eg. sickle cell anemia , heterozygous for sickle cell have a sickle cell trait

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define locus

fixed position of a gene on a chromosome

alleles of a gene are found at the same locus on each chromosome

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define homozygote

an organism that carries 2 copies of the same allele

eg BB, bb

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define heterozygote

an organism that carries 2 different alleles

eg. Bb

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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

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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

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some genes have multiple alleles

eg. blood group

caused by specific antigen- A, B, O

 IO = recessive

IAIB= co-dominant

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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

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what are the assumptions of HW?

-large population

-no migration

-no mutations

-mating must be random

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equation for predicting allele freq



p= frequency of dominant allele

q= frequency of recessive allele

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equation for predicting genotypye freq.



p2= frequency of homozygous dominant

2PQ= frequency of heterozygous

q2= frequency of homozygous recessive

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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

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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

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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

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explain speciation

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

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