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Transmission Genetics

How traits are passed down from generation to generation.

Transmission of genes and the phenotypes which come from those genes

The phenotype determines how the individual interacts with the world, and it is the phenotype that is subject to natural selection.

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Gregor Mendel 1856 -1863 monk, Czech Republic Studied 7 traits in pea plants, Pisum sativum

Established basic rules of transmission genetics Good science, but ignored for >30 years

Why peas?Many varieties with contrasting traitsSelf-pollinating, with true-breeding varietieseasy to snip parts to cross pollinateNeed little space, produce lots of offspring

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His experiments would not have worked out except:

1) He chose traits that were all dominant or recessive (“contrasting traits”)

2) He chose traits that were all located on different chromosomes (pea plants

have 7 chromosomes)

Pretty amazing since he had no idea how these traits were passed on – he called them “unit factors”

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Mendel’s unit factors we now call genes. The two versions of each gene are called

alleles. If an organism has two copies of the same

allele is it said to be homozygous.True breeding

If an organism has different alleles of the same gene it is said to be heterozygous.

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An allele is dominant, if it is expressed whenever that allele is present.

A recessive trait is one that is hidden by another. It is only expressed when the allele is

homozygous, that is 2 copies of it and none of another allele.

A dominant allele masks, or hides a recessive allele.

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The Genotype is all the genetic information the individual has for a particular trait. In a diploid organism, that means two copies.

Those traits that are expressed: can be seen (physical traits) or measured (chemical traits) are the individual’s phenotype.An individual heterozygous for a trait may only

show the effect of one allele (a dominant one) and not the other recessive one.

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Breeding two different, genetically distinct organisms is called cross breeding or crossing.

The offspring of such crosses are called hybrids.

The parents are called the parental or P generation

The offspring of these parents are called the F1 generation (first filial)

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Dominant traits are written with capital letters. Recessive traits are written with small letters.

P = purple pigment (Purple flowers) p = no pigment (white flowers)

PP = ? Pp = ? pp = ?

PP and Pp = purple flowers pp = white flowers

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Homozgous individuals are true breeding (produce same trait when self-pollinated)

What would we get if we crossed a homozygous purple flower (PP) with a homozygous white flower (pp)?

To find out, we can use a Punnett square – named after Reginald Punnett

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(PP)

P P

p Pp Pp

p Pp Pp

Parent 2

Parent 1

ppGametes made; each has a 50% chance of getting an allele from either homolog.

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Results: the offspring are all heterozygous Genotype of all is Pp P is dominant to p, so all have purple

flowers; the white flower phenotype is masked (p, white, is recessive)

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What would happen if we crossed members of the F1 generation?

P p

P

p

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Genotypes: 1: 2 : 1 PP : Pp : pp

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If you come upon a plant with purple flowers, it could be because the plant has a genotype of PP, homozygous for purple; OR it could be heterozygous, Pp, with the white allele masked.

How do we find out if it is homozygous or heterozygous ?

We could do a back cross or test cross – breed the individual with a homozygous recessive individual.

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Why breed it with a homozygous individual?Because it is the only parent that we can be

sure of its genotype from its phenotype!PP is purple, but so is Pp (that’s our question)Because the only way a recessive trait can be

seen is if there is no dominant trait hiding it, then a white flower MUST have the genotype pp which means you can tell what the genotype is from the phenotype.

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Genotypic ratio 1:1 Pp : pp

If the purple plant was homozygous, the F1 generation would all be purple - Pp

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But, not all traits show simple dominant-recessive relationships. There is also partial dominance where both traits are expressed.

Some traits show incomplete dominance.Snap dragons have genes for red flowers (R1)

and white flowers (R2). A heterozygous flower (R1R2) would be

Pink!This type of trait gave early scientists the idea that traits blended in offspring of different individuals.

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Notice that both traits are given capital letters, and the F2 generation shows a 1:2:1 ratio of phenotypes as well as genotypes.

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Other traits show Codominance where both alleles are equally expressed.

Blood types: A B O blood groups

Glycolipids on the cell membrane of RBCs. IA, IB and i (A and B are codominant; O is recessive)

IAIA = Type A IBIB = Type B

IAi = Type A IBi = Type B

IAIB = Type AB ii = Type O

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Mendel’s Crosses Showed: Principle of segregation: each sexually

reproducing organism has two genes for each characteristic, and these two genes segregate or separate during the production of gametes.

Principle of independent assortment: traits which lie on different chromosomes are passed on independently of each other.

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(PP)

P P

p Pp Pp

p Pp Pp

Parent 2

Parent 1

ppGametes made; each has a 50% chance of getting an allele from either homolog.

Mendel’s Law of Segregation

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Mendel’s Independent Assortment

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With these rules in mind, we can cross individuals that have two different traits.

Dihybrid (vs. monohybrid) cross.

We can cross peas that have green pods (G) which are inflated (I) with peas that have yellow pods (g) which are constricted (i).

GG II X gg ii = Gg Ii

parents F1

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The only “trick” to a dihybrid cross is setting up the Punnett square.

GgIi →

GI, Gi, gI, gi

Like making a snack – take one of each

Chips: Fritos or Doritos

Fruit: apple or a peach

candy bar: snickers or butterfinger

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GI Gi gI gi

GI GGII GGIi GgII GgIi

Gi GGIi GGii GgIi Ggii

gI GgII GgIi ggII ggIi

gi GgIi Ggii ggIi ggii

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Genotypes :

1 GGII : 2 GgII : 2 GGIi : 4 GgIi

1 GGii : 2 Ggii 1 ggII : 2 ggIi

1 ggii

Phenotypes: 9 Green inflated

3 Green constricted

3 Yellow inflated

1 Yellow constricted

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Sex-Linked traits Some genes carried on the X chromosome

are missing from the Y chromosome. These traits show up in different ratios in

males and females and are called sex-linked traits

Males are said to be hemizygous for these traits since they can only have one gene and a recessive gene will always be expressed.

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Color blindness is carried on the X chromosome (X’) Normal color vision (X)

X Y The females all have

X XX XY normal color vision, but

half the males are color

X’ XX’ X’Y blind.

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X’ Y Here half the females

X XX’ XY and half the males are

X’ X’X’ X’Y color blind.

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What happens when two traits are located on the same chromosome?They tend to be passed on together – this is

called genetic linkage.

Can these two traits be inherited separately?

The likelihood that two genes on the same chromosome will be inherited separately depends on the distance between them.

They can be separated during “crossing over” that occurs during Prophase I of meiosis.