How to do genetics crossings How to do genetics crossings

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SIMPLE MONOHYBRID CROSSES INVOLVING A SINGLE PAIR OF CONTRASTING ALLELES OF A

SPECIFIC TRAIT•DOMINANT AND RECESSIVE•INCOMPLETE DOMINANCE

•CO-DOMINANCE

Introduction (dominance & Introduction (dominance & recessive characteristics)recessive characteristics)

Study the information given in the Study the information given in the question. Look for the following aspects:question. Look for the following aspects:1.1.The trait with the contrasting allelesThe trait with the contrasting alleles2.2.Which allele is dominant and which one Which allele is dominant and which one is recessiveis recessive3.3.The genotype of the two parents The genotype of the two parents (homozygous or heterozygous)(homozygous or heterozygous)4.4.The phenotype of the two parentsThe phenotype of the two parents5.5.What is the extra question? (ratio, What is the extra question? (ratio, probability, numbers) probability, numbers)

QUESTIONQUESTIONFlies with heterozygous grey bodies were crossed with

flies with black bodies. Grey bodies were dominant over black bodies. Use the letters G and g to represent a genetic cross to show the F1 genotypes and phenotypes.

1. Trait:

2. Contrasting alleles:

3. Dominant:

4. Recessive:

5. Genotype of Grey parent:

6. Genotype of black parent:

Colour of bodiesGrey and black

Grey (G)

black (g)

heterozygous

Not given

Gg

When is a recessive characteristic expressed?Can always be deducted: There is only one combination of

recessive alleles that will make the recessive characteristic to be visible/functional/expressed …

the homozygous recessive alleles (two recessive alleles)

In this case: gg 4

Using a template to do a crossingUsing a template to do a crossing

Have to provide the examiner the Have to provide the examiner the following:following:Phenotype and Genotype of the two Phenotype and Genotype of the two parents parents meiosis and the gametes formedmeiosis and the gametes formed Fertilisation and the punnett squareFertilisation and the punnett squareFF11 ratio of genotypes and phenotype ratio of genotypes and phenotype

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General Marking guidelines

Gametes G G

g Gg Gg

g Gg gg

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√

Memo for questionP1 Phenotype Grey body x Black body √

Genotype Gg x gg √

Meiosis

Correct gametes √

F1 Phenotype: 3 Grey : 1 black √

Genotypes: 3 Gg : 1 gg √

F1 & P1 : √

Meiosis and fertilisation: √ max 77

gametes G g

g Gg Gg

g Gg gg

Some important notesSome important notes In answering the questions about a genetic crossing always use this format/template described in the previous slide. Use a punnett square to do the crossings of the gametes – less confusing Use the generations (P1 and F1) as well as the processes (meiosis and fertilisation) in the correct places – can obtain two marks even if your crossing is incorrect. This format/template is applicable for all crossings – also for dihybrid crossings

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No dominance or recessive alleles. The homozygous offspring or two different parents have two contrasting characteristics. The heterozygous offspring or parents have an characteristic which is in between (intermediate) to the two contrasting characteristics of the different homozygous organisms Any information about an intermediate/in between value refers to incomplete dominance. Use for both contrasting alleles capitals e.g. R = Red and W = white. RW = pink (intermediate colour).Use the same template/format for the crossing.

Incomplete dominance question

A red flower snapdragon is crossed with a white flower snapdragon. Use the symbols R for red

flowers and W for white flowers.

The heterozygous for this trait is a pink flower. Use a genetic crossing to illustrate the possible

genotype and phenotype ratio of the F1.

Memo for questionP1 Phenotype Red flower x White flower √

Genotype RR x WW √

Meiosis

Correct gametes √

F1 Phenotype: All pink flowers√

Genotypes: all heterozygous / RW√

F1 & P1 : √

Meiosis and fertilisation: √ max 711

gametes R R

W RW RW

W RW RW

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Both the contrasting alleles for a trait are equally expressed. The homozygous offspring or two different parents have two contrasting characteristics. In the heterozygous offspring or parents both contrasting characteristics appear separately / are equally expressed.For example: Inheritance of blood groups A and B Genotype for blood groups A and B = IA, IB

Note: Blood group O (genotype = ii) is recessive to both blood group A and B.Another example: cattle with red fur coat crosses white fur coat, produce calves that are roan.

Co-dominance question

A man with a homozygous blood group A is crossed with a woman with a homozygous blood group B. Use a genetic crossing to illustrate the possible ratio of the phenotype and genotype of

the offspring.

Memo for questionP1 Phenotype Blood group A x Blood group B √

Genotype IAIA x IBIB√

Meiosis

Correct gametes √

F1 Phenotype: All blood group AB√Genotypes: all IAIB √

F1 & P1 : √

Meiosis and fertilisation: √ max 7

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gametes IB IB

IA IAIB IAIB

IA IAIB IAIB

Co-dominance question

A bull with a red fur coat (R ) is crossed with a cow with a white fur coat (W ) and produces

calves that are roan. Use a genetic crossing to illustrate the possible ratio of the phenotype and

genotype of the offspring.

Memo for questionP1 Phenotype Red fur coat x white fur coat √

Genotype RR x WW√

Meiosis

Correct gametes √

F1 Phenotype: All calves roan√

Genotypes: all RW √

F1 & P1 : √

Meiosis and fertilisation: √ max 716

gametes W W

R RW RW

R RW RW

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Introduction – sex chromosomes•Females have two X chromosomes•If a woman has a rare recessive allele on the one X chromosome, she is likely to have a dominant allele on the second chromosome •Males have a X and a Y chromosome•The Y chromosome is much shorter and carries a different set of genes•If a man has a rare recessive allele on his X chromosome, his phenotype will show it because there is no second X chromosome to carry a dominant allele that could hide it.

Sex-linked crossing question

Two examples prescribed, namely, haemophilia and red-green colour blindness. Both are recessive sex-linked genes carried on the X chromosomes.

Haemophilia

An unaffected father is married to a heterozygous mother for haemophilia. Use a genetic cross to illustrate the possible genotype and phenotype ratio of the offspring. Use the symbols XH, Xh

Memo for questionP1 Phenotype Unaffected male x heterozygous female √

Genotype XH Y x XH Xh √

Meiosis

Correct gametes √

F1 Phenotype: 2 unaffected females, 1

unaffected male, 1 affected male√Genotypes: 1 homozygous and 1 heterozygous

female, 1 XH Y, 1 Xh Y male, √F1 & P1 : √

Meiosis and fertilisation: √ max 7

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gametes XH Xh

XH XHXH XHXh

Y XH Y Xh Y

Some notesMales are affected or unaffected. Only one X

chromosomeFemales can be a carrier. Have heterozygous alleles

on the two X chromosomes.Females very seldom have the recessive genes on

both X chromosomes.

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