MichLim_MendelianGenetics_Mugsheet_2013 1

MENDELIAN GENETICS MUGSHEET

I . Definitions to Memorise Either asked for explicitly, or required when “explaining phenomenon” in Punnett problems

CA

RR

IER

One who carries the recessive al lele but appears just as normal as homozygous dominant genotype’s phenotype TR

UE

B

RE

ED

homozygous

CO

D

OM

INA

NC

E where one gene’s 2 alleles are

different, but equally dominant and cannot mask each other. Both al leles are expressed in heterozygote’s phenotype.

INC

OM

PLE

TE

DO

MIN

AN

CE

Where one allele is not completely dominant over the other, creating a new phenotype for the heterozygous individual, not expressed in either parent

MU

LTIP

LE

ALL

ELE

where single characteristic appears in >2 al leles, of which only 2 occupy same gene locus in homologous chromosomes in each individual. PO

LYG

EN

IC

INH

ER

ITN

CE

≥2 gene pairs (different loci) affect single phenotype and have additive effect

I I . Variation Answer Scheme

CONTINUOUS DISCONTINUOUS Shape

How do you tell?

• Hand width follows normal distr ibution curve

• No distinct phenotype; there are intermediate forms of Hand width

• There are few, dist inct phenotypes

• No intermediate forms of blood group

Factors • is a polygenic trait, influenced by many genes, each having a little overall additive effect on hand width

• Influenced by environment: factors like space and nutrients affect growth and differential expression of genes that code for the structure

• 1 ALLELE PAIR • Not influenced by env

E.G. Height, Seed Size, Greenness of plants, Skin shade Tongue rolling, natrl hair color Benefits of Variation

• Adaptation to env: if conditions change, more variations available for natural selection to act on.

• Increase survival: accumlatn of favourable heritable traits increases fitness Mutatn: ultimate variation source

• Creation of new allele due to change in DNAàmore genotypes and phenotypesànew possible combinations

• Gene shuffling: only different combinations of existing alleles.

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I I I . Mendel’s Laws

INDEPENDENT ASSORTMENT SEGREGATION

Each al lele pair separates independent of others.

Each al lele pair separates into 2 alleles, each in a different gamete.

Metaphase I : arrangement of chromosomes in each pair of homologous chromosomes is independent of other chromosomes.

Anaphase I I : sister chromatids separate to opposite poles of cell

Anaphase I : chromosome of 1 homologous pair separates independently of other pair.

Signif icance: Diff recombination of maternal/paternal chromosomes

Child receives 1 allele from each parent.

IV. Intuition

Cross Ratio Remarks Pedigree Analysis Aa X aa 1:1 Test Cross: test

unknown with recessive, if recessive phenotype appears in offspring, unknown is Aa

1. Is it Autosomal or Sex-l inked?

Autosomal Sex-linked

Around 50/50 between females and males

Most Males Affected

X: most males affected

Y: only males affected

AA X aa All Dom

2. Recessive or Dominant?

Aa X Aa 3:1 Dominant X-linked Dominant -nil- AB X AB 1:2:1 Codominance/Inc

omplete dominance ratio

Each affected kid has affected parent

Affected daughter with affected dad

AaBb X aabb 1:1:1:1 Test Cross Recessive X-linked Recessive AaBb X AaBb 9:3:3:1 Affected kid with

healthy parents -same as dom- OR Affected son w/ affected mum

V. Punnett Square

5 parts (A) KEY, (B) PARENTAL, (C) F1, (D) Genotype/Phenotype Ratios, (E) Conclusion/ANS • Write all 5 parts for prediction of genotypes questions too. • When writing explanations, write key at start if you refer to the letters/alleles

Trick Qn (after calculating probability of offspring’s phenotype in Punnett…): Why al l daughters express only this phenotype?

• 1m for explaining sample size too small ; • 1m for explaining her phenotype + where each of the alleles came from

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Punnett Square Protocol

• Need not write out repeated gametes. Punnett Square does not have to be 2x2 or 4x4. E.g.(à)

• The “Parent” line in Parental Generation is optional, but compulsory for Sex-linked.

Monohybrid (NNxnn) Dihybrid

Sex Linkage Co-dominance/Multiple Allele Series

Incomplete Dominance/ Codominance [Only difference is F1Phenotype]