principles of heredity

Principles of

Heredity

Chapter 10

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Gregor Mendel

• Generally considered the ‘Father of Modern Genetics’

• Worked with pea plants, keeping careful records of his experiments

• Used statistical analysis to establish several important genetic principles

Mendelian Genetics

• Things he knew before he started:– How to control pea

plant reproduction– That he had true-

breeding plant strains– That hybrids between

these strains did NOT breed true

Mendelian Genetics…

• Things he didn’t know:– What DNA or

chromosomes were– Anything about mitosis

or meiosis– That traits in hybrids

did not always ‘blend’ as was the idea of the time

Mendelian Genetics…

• What he figured out:– Blending of two distinct traits in the parents did not

always occur in the hybrid offspring– Each kind of inherited feature in an organism is

controlled by 2 factors that behave like distinct particles

– That some of these factors can mask others (that is, some are dominant while others are recessive)

– The Principle of Segregation and The Principle of Independent Assortment

The Principle of Segregation

• Alleles – alternate gene forms – are located on corresponding loci on homologous chromosomes

• During gamete formation in meiosis, homologous chromosomes separate (when?)

• During sexual reproduction, offspring receive one of these homologous chromosomes from each parent

Terms to review

• Dominant / Recessive

• Gene / Allele

• Monohybrid cross / Dihybrid cross

• Homozygous / Heterozygous

• Phenotype / Genotype

• Punnett square / Probability

• Test cross

Monohybrid cross practice

1.) A TT (tall) pea plant is crossed with a tt (short) pea plant

Monohybrid cross practice…

2.) A Tt pea plant is crossed with a Tt pea plant.


3.) A heterozygous round seeded pea plant (Rr) is crossed with a homozygous round seeded pea plant (RR).


4.) A homozygous round seeded pea plant is crossed with a homozygous wrinkled pea seeded plant.


5.) In pea plants purple flowers are dominant to white flowers. Cross two white flowered plants.


6.) A white flowered pea plant is crossed with a pea plant that is heterozygous for the trait.


7.) Two pea plants, both heterozygous for the gene that controls flower color, are crossed.


8.) In guinea pigs, short hair is dominant over long hair. Show the cross for a pure breeding short haired guinea pig and a long haired guinea pig.


9.) Show the cross for two heterozygous guinea pigs.What percentage of the offspring will have short hair? ________What percentage of the offspring will have long hair? _______


10.) Two short haired guinea pigs are mated several times. Out of 100 offspring, 25 of them have long hair. What are the probable genotypes of the parents? Show the cross to prove it!

Dihybrid cross practice

IN PEAS:R = round T = tall Y = yellow peas P = purple flowers

r = wrinkled t = short y = green peas p = white flowers

1.) Homozygous tall, round parent X pure short, wrinkled parent

Dihybrid cross practice…



2.) Heterozygous for both height and flower color parent X short, white flowers parent




3.) Green peas, short plant X Heterozygous for yellow peas, homozygous for tall parent




4.) Heterozygous round, green peas X wrinkled peas, Heterozygous yellow peas




5.) Both parents heterozygous for height and flower color

The Principle of Independent Assortment

• Another of Mendel’s ideas

• Explains the results of these types of dihybrid crosses

• Each different trait is inherited independently from the other

• Now we know that this is due to meiosis – homologous chromosomes separate independently (again, when?)

Probability Rules

• The product rule:– Predicts the combined probability of 2

independent events– If two or more events are independent of each

other, the probability of both occurring is the product of their individual probabilities

– Example: coin toss – heads two times in a row• ½ X ½ = ¼ or one chance in 4

– Also: Bb X Bb parents – producing a bb child• ½ b X ½ b = ¼ or one chance in 4

Probability Rules…

• The sum rule– Predicts the combined probabilities of

mutually exclusive events– If there is more than one way to get a result,

we combine the probabilities by summing– Example: Bb X Bb parents chance of Bb child

• 2 possibilities: B egg + b sperm; b egg + B sperm• B egg (½) X b sperm (½) = ¼ • And b egg (½) X B sperm (½) = ¼ • Then … ¼ + ¼ = ½

Probability Rules…

• Most important –– Chance has no memory!Chance has no memory!– If events are truly independent, past events

have no influence on the probability of future events…

– Even though we don’t like this idea – ‘my luck is bound to change…’

Things Mendel didn’t know

• Linked genes - inherited together because they are located on the same chromosome– Linked genes do not undergo segregation or

independent assortment– The rates of crossing over can be used to determine

the relative positions of genes on a chromosome– Higher crossing over rates indicate greater separation

of genes on a chromosome– Each %age of crossing over rate = one map unit

Things Mendel didn’t know…

• Sex chromosomes:– Female = XX - Male = XY– Male produces the sex determining gamete– In humans, Y chromosome has the SRY gene

(sex reversal gene on the Y) – this acts as a genetic switch to cause testes to develop

– Developing testes produce testosterone which determines other sexual characteristics

– Everyone has at least one X – female is the ‘default’ sex… need a Y to develop as a male


• Sex-linked genes:– Located on the X chromosome only– Include genes for color perception and blood

clotting – things all humans need– Females get two copies – can be either

homozygous or heterozygous– Males only get one copy – they are

hemizygous– Defects in these traits arise more in males

than in females


• Dosage compensation:– Makes equivalent the female’s 2 ‘doses’ of the

genes on the X chromosome to the male’s 1– One X chromosome in each female cell is

inactivated – called a Barr body– Individuals with heterozygous X-linked genes

will often have a variegated phenotype as random X chromosomes are inactivated in the body

Calico cats – an example of X chromosome inactivation in action


• Incomplete dominance– The phenotype of the

heterozygous individual is a blending of the two genes


• Codominance:– The phenotype of the heterozygous individual

expresses both genes, but without blending

ABO Blood Types:

A and B alleles are codominant to each other

Both are dominant to type O allele


• Multiple alleles:– Three or more alleles exist in the population,

even though each individual only has two– Examples:

• ABO blood types in humans• Coat color in rabbits


• Pleiotrophy– One gene with many effects– Often found in genetic diseases– Example:

• Cystic fibrosis in humans• Homozygous individuals produce abnormally thick

mucus in many body systems


• Epistasis– The presence of one

allele can prevent or mask the expression of a gene at another loci

– Example:• Coat color in Labrador

retrievers• Pigment gene is either B

(black) or b (brown)• Recessive ee blocks the

expression of either

Black Brown YellowBBEE bbEE BBee

BbEE bbEe Bbee

BBEe bbee

BbEe


• Polygenic inheritance– Mulitple independent pairs of genes have

similar and additive effects on the phenotype– The phenotypes in a population will generally

show a normal distribution curve– Examples:

• Human skin and eye color


• Environmental interaction– Genetically identical individuals show different

phenotypes based on environmental factors– Example:

• Human height and intelligence

– Nature versus nurture questions• Problems with experimental methods to answer

these questions in humans….

principles of heredity

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