Mendel Genetics and Punnett Squares- NOTES

Terminology • Heredity- the passing of characteristics from parents to offspring• Genetics- the branch of Biology that focuses on heredity• Cross- combining traits from two parents by breeding (sexual reproduction)• Trait- characteristic that can be passed down to a child from their parents carried on their chromosome (ex. Eye

color, hair, height, etc) The Origin of Genetics• Gregor Mendel

– Austrian monk – Studied theology, science and mathematics– Noticed patterns in the pea plants he was growing– Mendel crossed pea plants and studied the genetic turn-outs – Was the first to develop rules that accurately predict patterns of heredity– Father of genetics

• Why study pea plants?– They reproduce quickly– Easy to see their traits– Flowers, because of their structure, are easy to control reproduction (by self pollinating)

• Self pollination occurs in plants that have both male and female reproductive structures on the same flower/plant. The plant is able to fertilize itself

• Mendel discovered two types of organisms– Purebred- they receive the same traits from each parent (ex. Purebred yellow Labrador)– Hybrid- receive different traits from each parent (ex. Labradoodle)

Theory of Heredity • Based on Mendel’s observations:

– For each trait, an individual has two copies of a gene- one from mom and one from dad– There are alternative versions of genes called alleles– When 2 different alleles occur together 1 of them may be expressed (visible) while the other may have no

effect on appearance– Each gamete (sperm/egg) contributes one allele

• Mendel’s law of segregation– States that the members of each pair of alleles separate when gametes are formed

• Mendel’s law of independent assortment– States that pairs of alleles separate independently of one another during meiosis (gamete formation)

Monohybrid Crosses Mendel used monohybrid crosses to study characteristics of pea plants Monohybrid cross- a cross that involved one pair of contrasting traits

Ex. Crossing a round seed of a pea plant with a wrinkled seed of pea plant.

Terminology• Alleles- alternative versions of genes

– The gene for hair color can have multiple alleles (blonde hair allele, brown hair allele, etc)

P generation- the parental generation of true-breeding individuals

F1 generation- first filial generation; offspring of the P generation resulting from crossing of

contrasting traits

F2 generation- second filial generation; offspring from the self pollination of the F1 generation

Maternal chromosome that came from the egg of this person’s mother

• Dominant- the expressed (visible) trait– Allele written as a capitol letter (B, R, G, etc)– It will typically mask or cover up recessive traits

• Recessive- the trait that is not expressed or observable– Allele written as a lowercase letter (b, r, g, etc)

Problem If “B” is brown and “b” is blonde:

• What trait would be visible for:– BB?– bb?– Bb?

Terminology Continued• Homozygous- when two alleles of a particular gene are the same

– (PP), (pp), (RR), (rr), (VV), (vv), etc• Heterozygous- when two alleles of a particular gene are different

– Only the dominant allele is expressed while the recessive is present, but not expressed– (Pp), (Rr), (Vv), etc

Capitol letter is always written first! • Genotype- the set of genes or alleles an individual has for a trait

– think “genes” and “letters”– (Rr), (RR) or (rr)– (Vv), (VV) or (vv)

• Phenotype- the physical appearance of a trait– Think “face” and “colors”– White, purple, red, blonde, freckles, blue eyes, etc

Practice:(assume that P is purple and p is white)(Pp):

What is the genotype?What is the phenotype?Is it homozygous or heterozygous?

(pp):What is the genotype?What is the phenotype?Is it homozygous or heterozygous?

(PP):What is the genotype?What is the phenotype?Is it homozygous or heterozygous?

Punnett Squares• Punnett square- is a diagram that predicts the outcome of a genetic cross by

considering all possible combinations

Practice:• Ex. Having freckles (F) is dominant to not having freckles (f).

Parent 1 is heterozygous for freckles and Parent 2 is also heterozygous

Paternal chromosome that came

from the sperm of this person’s

Heterozygous- only the dominant allele is expressed

Homozygous dominant- both chromosomes have identical, dominant alleles

Homozygous recessive-both chromosomes have identical, recessive alleles

Genotype of parent one

Genotype of parent two

Genotype of parent one

Genotype of parent two

1 2

3 4

Box 1 is homozygous for….Box 2 and 3 is…..Box 4 is….

Once a punnett square is solved you can determine ratios of possible outcomes• Look at the combinations INSIDE the punnett square only• Genotypic ratio asks for the combination of alleles/letters• Phenotypic ratio asks for the combination of physical traits• Always written homozygous dominant, heterozygous,

homozygous recessive • Numbers in the ratio MUST add up to the number of boxes in a punnett square !!!

Practice • Ex. A homozygous yellow pea plant (Y) is dominant over a homozygous green pea plant (y).

Determining Unknown Genotypes• When a phenotype is dominant but the genotype is not known, a test cross is performed.

– You cross the unknown genotype with a homozygous recessive individualWhat does that mean?

- You can physically see that the pea plant is yellow, but you don’t know if they are (YY) or (Yy). - So, you cross the yellow pea plant with a recessive pea plant (yy) that is green.

If the offspring are all yellow the genotype is (YY)If the offspring are ½ yellow and ½ green the genotype is (Yy)

• Doesn’t make sense? Use the Punnett Square and work backwards!

Genetics Basics - Vocabulary

genetics- study of heredity (probability)trait- characteristic that can be passed down to a child from their parents carried on chromosomes (Ex.- eye color, hair, height, etc)

Looking at the punnett square above determine the:Genotypic Ratio: __________

Phenotypic Ratio: _________

- What’s the genotype of all offspring?

-What’s the phenotype of all offspring?

-What’s the genotypic ratio?

-What’s the phenotypic ratio?

autosomes —chromosomes that don’t determine gender (in humans, chromosome pairs 1-22) sex chromosomes —chromosomes that determine gender (in humans, the 23rd pair, XX or XY)

Gregor Mendel- Father of modern geneticsAustrian monk that first developed a way to accurately predict patterns of heredity

purebred- they receive the same trait from both parents (ex. purebred yellow lab)

hybrid- receive different traits from each parent (ex. yellow x chocolate; 3 black, 3 white, 1 butterscotch)

allele- different form of the gene; copy of a gene2 Types of Alleles:

1. dominant allele- trait being expressed (capital letter: T= tall)2. recessive allele- trait being masked by dominant allele (lower case letter: t= short)

genotype- both genes or alleles (What the genes/alleles (“letters”) are)1. homozygous- when an individual has 2 copies of the same allele Ex. TT or tt2. heterozygous- when an individual has different alleles (one copy of each) Ex. Tt

Why do two letters represent the genotype?There are 2 parents (one allele from mom, one allele from dad)

phenotype- outward expression (what characteristic do you see – “words”); physical appearancetall vs. short; brown eyes vs. blue eyes

Theory of Heredity- based on Mendel’s observations1. For each trait, an individual has two copies of a gene- one from mom, one from dad2. There are alternative versions of genes- alleles3. When 2 different alleles occur together, 1 of them may be expressed while the other may have no effect on appearance. (dominant, recessive)4. Each gamete (sperm, egg) contributes one allele.

Law of segregation – the members of each pair of alleles separate when gametes are formed

Law of independent assortment – pairs of alleles separate independently of one another during gamete formation (if they are located on different chromosomes or far apart on the same chromosome)

self-pollination – occurs in plants that have both male and female reproductive parts on the same flower/plant (monoecious); the plant fertilizes itself; pea plants have monoecious flowers

cross-pollination – occurs in nature in plants that have only male or female reproductive parts on the same plant (dioecious); pollen from the male plant must reach the pistil of the female plant; what Mendel did when he transferred pollen from one plant to another by hand P generation – the parental generation of true-breeding individuals

F1 generation – first filial generation; offspring of the P generation resulting from the crossing of contrasting traits

F2 generation – second filial generation; offspring from the self-pollination of the F1

monohybrid cross – cross involving one trait (Tt x Tt)

test cross – the crossing of a phenotypically dominant individual of unknown genotype with a homozygous recessive individual to determine the unknown genotype (tt x T-)

Punnett square – diagram used to predict the outcome