Genetics Heredity Mendel and Punnett Squares Standard S

Genetics &Heredity: Mendel and Punnett Squares

Standard: S 7 L 3 Students will recognize how biological traits are passed on to successive generations. a. Explain the role of genes and chromosomes in the process of inheriting a specific trait.

Heredity • The passing of traits from parents to offspring. • Traits- The characteristics of an organism. **(any description)

Alleles • A different form of a gene. • One allele may code

Parents You get one allele from mom and one from dad.

Genetics The study of how traits are inherited through the interactions of alleles.

Gregor Mendel • The first to study how traits were passed from one generation to the next. • He studied pea plants.

Mendel’s Experiments • Mendel crossed a pure breeding white flowered plant with a pure breeding purple flowered plant. • He got all purple flowers in the offspring. • BUT, then he crossed two of the second generation purple flowers and he got ¾ purple flowers and ¼ white flowers!

So…. what does that mean? From the results of his second cross, Mendel determined that one allele was dominant and the other allele was recessive.

Dominant vs. Recessive • Dominant Alleles- An allele that covers, or dominates another allele. **the dominant allele is ALWAYS expressed when a dominant allele is present. • Recessive Allele- An allele that is covered, or hidden by another allele. **TWO recessive alleles must be present for the recessive allele to be expressed.

Representing Alleles • Letters! • Upper case= dominant alleles. (R) • Lower case =recessive alleles. (r)

Genotype vs. Phenotype • Genotype - An organism’s genetic makeupcombination of alleles that determine the organism’s phenotype. • Phenotype - Outward

Punnett Squares • A tool to predict the probability of certain traits in offspring. • Punnett squares show the different ways alleles can combine.

Homozygous • If the two alleles that you get from your mom & dad are the same, it is homozygous (RR, rr)

Heterozygous • An organism that has two different alleles for a trait is called heterozygous. (Rr)

Example: • The tongue rolling allele is R. • R is dominant • r is recessive

• Your mom is homozygous dominant for the tongue rolling gene. • Her genotype is RR

• Your dad is heterozygous for the tongue rolling gene. • His genotype is Rr

Probability • What is the probability that you will be able to roll your tongue? • Do a Punnett Square!

Making a Punnett Square • To set up a Punnett Square, draw a large square. • Then, divide it into four equal sections.

Dad’s Alleles go on top Mom’s Alleles go on the side

R R R Eggs r Sperm

R r Sperm R RR Rr Eggs

Your genotype could be: – 50% RR- Homozygous Dominant – 50% Rr- Heterozygous Your phenotype would be: – 100% Tongue Roller – 0% No Tongue Roller

Hybrid vs. Purebred • Hybrids=Heterozygous- Have two different alleles for a gene: Rr • Purebreds=Homozygous- Have the same allele for a gene: RR or rr

Let’s Do Another One! • The Big Nose Allele is N. • Big Nose is N • Little nose is n

• Mom is Heterozygous for Big Nose: Nn • Dad is Heterozygous for Big Nose: Nn • What is the chance you’ll have a big nose? Do the genetic cross to find out!

N n Sperm N NN Nn nn Eggs

What is the genotype? • 25% NN- Homozygous Dominant • 50% Nn- Heterozygous • 25% nn- Homozygous Recessive

What is the phenotype? • 75% Big Nose • 25% little nose

Incomplete Dominance When the offspring of two homozygous parents show an between” phenotype. “in-

Example: • Purebred white flowers • Purebred flowers • Makes “inbetween” pink colored flowers