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Genetics

Definitions

Inheritance the transmission of genetic information in the form of genes from one generation to

the next

Gene a length of the genetic material DNA that codes for the manufacture of a particularprotein. It can be copied and passed on to future generations e.g. amylase enzyme, collagen of

skin.

Chromosome a molecule (strand) of DN associated !ith proteins consisting of about "###

genes inherited as a unit e.g. the $ and % chromosomes.

Allele one or more alternative forms of a geneoccupying the same position on a chromosome

e.g. I, I&, i of human blood groups.

Genotype the genetic makeup of an organism, the allelesfor a particular gene that the organismhas e.g. Ii.

Phenotype thecharacteristics(physical features or chemical makeup) of an organism e.g.

blood group, cat coat colour. 'he enironment may affect the phenotype e.g. human skin colour,

height

Homozygous haing t!oidenticalalleles of a gene e.g. I&I&. 'he organism !ill be pure

breedingas it can only pass on one form of an allele

Heterozygous haing t!o differentalleles of a gene e.g. Ii. 'he organism !ill be not be pure

breedingas it can pass on one of t!o forms of an allele

Dominant allele the allele that is al!ays expressedin the phenotype if it is present e.g. II

genotype gies a blood group phenotype of group as I*i

ecessive allele + the allele that is only expressed in the phenotype in the absenceof the dominant

allele e.g. iigiing blood group, is recessie so Ii !ould be blood group.

Diploid nucleus !"n# a nucleus containing a pairof each chromosome. ll human body cells

are produced by mitosis and are diploid !ith $% chromosomes (- inherited from the father, -

from the mother)

Haploid nucleus !n# a nucleus containing one of each pairof chromosomes. ach human

gameteproduced by meiosiscontains - chromosomes to produce a zygote !ith /0 chromosomes

at fertilisation.

&utation a spontaneous change in a geneor a chromosome. It can be inherited by the

offspring e.g. 1aemophilia, colour blindness (gene) and Do!ns2s syndrome (extra chromosome).

3utation is a driing force of ariation and eolution but mutations are usually disadantageous.

Ionising radiation and mutagenic chemicalscan cause mutations.

''ic(le)cell anaemia is a mutation of the haemoglobin gene. 'he phenotype has a defect inhaemoglobin !hich causes red blood cells to flatten. 'hey can therefore carry less oxygen and the

person is lethargic, often thin and unfit. 'hese cells cannot be infected by the malarial parasite and

therefore the sufferer is immune to a common, often fatal, disease.

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'olving Genetics Problems

". 4ead carefully.

. 5nderstand the nature of the problem.

-. 4emember that parents and offspring hae t!o alleles (genes, possibly different) for each trait

(characteristic) and gametes hae one selected at random.

xample6

!ire+haired male %orkshire terrier is mated !ith a smooth haired female. ll the puppies hae !ire hair.7hen the pups are interbred amongst themseles, the litters produced total - !ire+haired and "- smooth puppies.

xplain the pattern of inheritance.

7hat !ould be the predicted result of crossing one of the 8"generation males !ith a smooth+haired female9

1o! could you determine the genotype of a !ire+haired female of the 8" generation9

%our thoughts are in italics6

!ire+haired male %orkshire terrier is mated !ith a smooth haired female (P generation). ll the puppies hae

!ire hair (wire is dominant, W>w where W is the allele fore wire-hair and wfor smooth. The parents are both

homozygous, WW and ww).

7hen the pups(F !eneration)are interbred amongst themseles, the litters produced total - !ire+haired and "-

smooth puppies (puppies are heterozygous Ww, inherited Wfrom male and wfrom female).

xplain the pattern of inheritance.

7hat !ould be the predicted result of crossing one of the 8"generation males !ith a smooth+haired female9

(smooth hair is re"essi#e homozygous)

1o! could you determine the genotype of a !ire+haired female of the 8"generation9

:olution6

;arental cross

; generation phenotype !ire x smooth

; generation genotype 77x !!; generation gametes 7,7x !,!

;unnet : 7! heterozygous

8"generation phenotype "##> !ire+haired

Interbreeding the 8"generation

8"generation phenotype 7ire x 7ire

8"generation genotype 7! x 7!8"generation gametes 7!, x 7,!

;unnet : 77, ?#> 7!, ?>!!

8generation phenotype @?> !ire+haired, ?> smooth.

'his corresponds closely to the actual ratio of - !ire+hair to "- smooth. 'he ;unnet :

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No! do the follo!ing problems6

". pure breeding &lack Babrador is mated !ith a !hite Babrador. 'he offspring are all black. ;redict the

outcome of interbreeding the 8"generation.

. If you !ere the pet+carer of a black male Babrador, ho! !ould you determine !hether it !as homozygous

or heterozygous9

-. Interdigital hair is dominant oer hairless digits. :ho! ho! t!o parents !ith interdigital hair could hae a

first child !ith hairless digits. 7hat is the probability that their second child !ill hae interdigital hair9

/. ;ollen from a red flo!ered ;ea ;lant !as used to fertilise a !hite flo!ered plant, 'he seeds !ere so!n andit !as found that "@ of the resulting plants !ere !hite flo!ered and "-/ !ere red flo!ered. Deduce the

;arental genotypes and explain the result of the cross.

' Codominance

8or some characteristics, one allele does not dominate another and both are expressedin the phenotype. n

example is roan coat colour in cattle !hich is formed !hen pure+breeding !hite is crossed !ith pure breeding

bro!n coat. 'he roan calf is heterozygousand has both !hite and bro!n hair.

5se 4 for 4ed coat and 4C for 7hite

4 4C (codominant).

Eomplete6

;arental cross

; generation phenotype

; generation genotype

; generation gametes

;unnet :

8"generation phenotype "##>

;roblem6

In the :napdragon, pink flo!er colour is due to codominance bet!een red and !hite flo!er colour. ;redict the

result of a cross bet!een t!o pink flo!ered plants. If a total of "-- seeds are produced, ho! many !ould you

predict !ould hae !hite flo!ers !hen germinated9

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' &ultiple Alleles

8or some characteristics, there are more than t!o alleles in the genome. ny indiidual possesses t!o genes but

there are a larger number of genotypes.

n example is 1uman &lood Froups.

'here are three alleles* IA* I+* i, IA- I+. i.

n indiidual can possess any t/oalleles !hich determine his phenotype, the +lood group* A* +* A+ or 0,

Fenotype ;henotype (&lood Froup) NoteII

Ii I * i

II& & I I&

I&I& &

I&i & I& * i

ii

'hree babies !ere born at the same time in the same !ard at a hospital and there !as some confusion as to the

parentage so each parent and child !as tested to determine their blood group.

'he result !as6

;arent &abyName 3ale &lood Froup 8emale &lood Froup Eode Name &lood Froup

4eek & &

4it & & E

Gech F

Dra! ;unnet :

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1he inheritance of gender,

Gender(male or female) is determined by chromosomal inheritance. 'he -rdpair of chromosomes $ and % is

called the heterosomes because they ary in appearance and genetic content.

2emalescontain t/o 3chromosomes, maleshae one 3 and one 4, the % chromosome determines maleness but

does not contain any alleles that match any genes on the $ chromosome.

Dra! a ;unnet :