5. Mendelian Genetics

Chapter 11

Meiosis Review• Mendel’s principles require offspring to inherit one copy of each gene from each parent.

• Formation of gametes must divide the chromosomes from each parent in half

Meiosis – Review• Gametes are reproductive cells (such as egg and sperm) that contain half the DNA of a normal cell for a given species.

• Only reproductive cells undergo meiosis.

Meiosis – Review

• Meiosis: result is that gametes contain one chromosome of each type from the parent.

Introduction: • Heredity- the passing of traits from parent to offspring

• Heredity is the key to the difference between species and variety within a population

• Genetics- study of heredity

• Early Ideas about Heredity–Blending Inheritance- the idea that offspring were always a combination of traits contributed by the parents (19th Century)

• Father of genetics • Czech Republic, born 1822• Monastery Center of Sci Learning (1851)

• In charge of garden at the monastery

Gregor Mendel

• Studied sexual reproduction of pea plants–Male – Pollen–Female – Ovules

Pollen producing stamen (male)

Ovule producing pistil (female)

• Pea plants typically self-pollinate–Pollen / Ovules from same plant

–Offspring of self pollination are genetically identical to parents

• Mendel found that self pollination could be prevented

• Mendel then cross pollinated plants with different traits and analyzed the results.–Pollen / Ovules from different plants

• Mendel limited his study to a few isolated traits that were easy to identify–Seed shape (round / wrinkled)–Seed color (green / yellow)–Plant height (tall / short)

Genes and Dominance• Mendel crossed plants with different char for the same trait. Ex: tall x short

• Hybrids – offspring resulting from cross breeding

• In the first generation of hybrids there was no blending of characteristics

• Hybrids all exhibited one trait. (all tall, all yellow, not mixed)

• Char of one parent seemed to have disappeared.

• Mendel concluded that individual factors do not blend

• Gene- a factor that controls a single characteristic–Gene for seed color–Gene for eye color

• Alleles- Different forms of a gene, producing various char.–Gene for seed color•Yellow allele / green allele

–Gene for plant height•Tall allele / short allele

• Homozygous- having two identical alleles for a given trait. (TT , tt, YY, yy)

• Heterozygous- having two different alleles for a given trait (Tt, Yy)

• Mendel also concluded that some alleles are dominant over others (recessive).–Tall dominant over short•If a tall plant is CP with a short plant, the offspring will have genes for both, but only tall will occur

Tall (TT) ---------------- Short (tt)

Tall (Tt)-------Tall (Tt)

• Genotype- the genetic make up for a particular char.–TT, Tt, tt

• Phenotype- the physical char. that is expressed as a result of genotype–Tall, Tall, Short

Segregation• The offspring from prior experiments were allowed to SP

• Naming generations:–Original Parents (P)–First generation (F1)–Second generation (F2)

Tall------------------------------Short

Tall-------Tall

Tall Tall Tall Short

P F1 F2

• F1 was allowed to self polinate producing F2

• Some of the F2 plants showed the traits that had disappeared in the F1 generation (recessive)

• Mendel assumed that the disappearance in F1 was because the dominant allele masked the recessive allele

• In order for recessive traits to reappear, they must have segregated from the dominant allele.

• In the F1 generation each plant received one allele from each parent, but only the dominant one was observed…

• Mendel believed that each generation receives two alleles for each gene, one from each parent

• Gene pairs are segregated in order to produce the gametes (sex cells: sperm, egg, ovum, pollen etc).

• Punnet Square is used to predict outcomes of segregation

Punnet Square- F1 cross

T T

t Tt Tt

t Tt Tt

Punnet Square- F2 cross

T t

T TT Tt

t Tt tt

Genotype Phenotype

Homozygous TT TallHeterozygous Tt TallHomozygous tt Short

The End

Independent Assortment• Mendel conducted experiments to assess whether traits segregated independently or were linked to each other.–Is the gene that determines seed color linked to the gene that determines seed shape?

• He performed a two-factor cross to test (examined two traits at once).–F1 : RRYY x rryy

•Homozygous round/yellow•Homozygous wrinkled/green

ry ry ry ry

RY

RY

RY

RY

ry ry ry ry

RY RrYy RrYy RrYy RrYy

RY RrYy RrYy RrYy RrYy

RY RrYy RrYy RrYy RrYy

RY RrYy RrYy RrYy RrYy

–As expected, the resulting F1 were heterozygous, exhibiting only the dominant traits (all round, all yellow).

–F1 was then allowed to SP

–F2 : RrYy x RrYy

•Heterozygous round/yellow

RY Ry rY ry

RY

Ry

rY

ry

RY Ry rY ry

RY RYRY RRYy RrYY RrYy

Ry RRYy RRyy RrYy Rryy

rY RrYY RrYy rrYY rrYy

ry RrYy Rryy rrYy rryy

–Phenotypes?•Mendel’s experiment prod:

9 Yellow Round

3 Yellow Wrinkled

3 Green Round

1 Green Wrinkled

–Two factor crosses consistently produce these ratios (9:3:3:1)

–Many of the characteristics that Mendel studied do sort independently

–Further studies showed that genes on the same chromosome are often linked.

Applying Mendel’s Principles• Genetics and probability:–Consistent results from Mendel’s work allowed for models to make predictions

–Probable outcomes could be calculated

Number of times a specific event

occurs

---------------------

The # of opportunities

Probability =

–Flip a coin (1/2 or 50%)–Roll a die (1/6 or 17%)–Punnet squares can be used to predict the probability of a particular outcome for given cross.

Yy x Yy

Y y

Y YY Yy

y Yy yy

• Punnet square shows ratios of possible outcomes.–1 in 4 or 25% chance of YY–2 in 4 or 50% chance of Yy–1 in 4 or 25% chance of yy

YY YyYy yy