Essential Question: How do organisms reproduce more organisms like themselves? All organisms are made out of cells The DNA inside of cells contains instructions to build proteins which determine the type of organism that will develop, how it will be built, and how it will function DNA is passed down from parents to offspring during reproduction In sexually reproducing organisms (ex. humans), the DNA comes from 2 parents and is passed down through special cells called gametes

NOTES 19 – Genetics: Mendel & Heredity

What is genetics? Genetics – the study of heredity, which means, how traits are passed from parents to offspring

Trait – a characteristic of an organism that can be inherited (passed from parents to offspring) Traits are controlled by genes EX. Eye color in humans is a trait controlled by genes which are passed from parents to offspring

Alleles – Different versions of the same gene EX. The eye color gene in humans has two alleles, blue and non-blue

Who was Gregor Mendel and why was he important? Gregor Mendel – Mendel was a Czechoslovakian monk who is considered the “father of genetics” for figuring out the basic rules of how traits are inherited by doing experiments with pea plants back in the 1850’s

Mendel’s Experiments Mendel did breeding experiments called test crosses A test cross is when you choose 2 parents and use them to create offspring to see how a specific trait (or traits) is passed down

Mendel’s Famous Experiment Mendel noticed that pea plants were either tall or short Mendel noticed that some tall plants when crossed always produced tall offspring and that some short plants when crossed always produced short offspring Mendel took a tall plant that always produced tall offspring and crossed it with a short plant that always produced short offspring

What happened? All the offspring were tall – what happened to the short trait? Mendel then took 2 of the tall offspring and crossed them This time, 75% of the offspring were tall, but 25% of the offspring were short – the short trait reappeared How can you get short plants from tall plants?

What Mendel Figured Out The Rule of Unit Factors Sexually reproducing organisms have 2 copies of each gene One gene comes from the sperm cell and one gene comes from the egg cell The genes can have different versions called alleles EX. For eye color, a person can have 2 non-blue alleles, 1 non-blue allele and 1 blue allele, or 2 blue alleles

What Mendel Figured Out The Rule of Dominance When an organism has 2 different alleles (like 1 blue and 1 non-blue), the trait that shows up is said to be dominant The allele that does not show up is said to be recessive

How can we predict what traits will appear in the offspring of parents? Thanks to Mendel’s discoveries, we can predict what traits offspring are likely to have as long as we know the traits of the parents

Punnett Square Punnett Square – a chart used to predict the traits of offspring Capitol letters are used to represent dominant alleles EX. T for tall allele in pea plants Lower case letters are used to represent recessive alleles EX. t for short allele in pea plants

Phenotype – The trait the organism shows EX. tall pea plant or short pea plant Genotype – The 2 alleles present in the organism for a trait EX. T T or T t or t t

Homozygous – The 2 alleles for a trait are the same EX. T T or t t Heterozygous – The 2 alleles for a trait are different EX. T t

Mendel’s Experiment Revisited Here is another look at Mendel’s experiment using some new terms Mendel examined the height gene in pea plants The height gene in pea plants has 2 alleles, tall (T) and short (t) The tall allele is dominant and the short allele is recessive

Mendel’s First Cross Mendel crossed a homozygous tall plant with a homozygous short plant T T x t t

Mendel crosses a homozygous tall plant (TT) with a homozygous short plant (tt)

The tall parent passes on either its first tall allele (T)…

…or its second tall allele (T) – either way, it always passes a tall allele

The short parent passes either its first short allele (t)…

…or its second short allele (t) – either way it always passes a short allele

Possible offspring 1 – the tall parent’s first tall allele with the short parent’s first short allele (T t)

Possible offspring 2 – the tall parent’s second tall allele with the short parent’s first short allele (T t)

Possible offspring 3 – the tall parent’s first tall allele with the short parent’s second short allele (T t)

Possible offspring 4 – the tall parent’s second tall allele with the short parent’s second short allele (T t)

Genotypic Percentage – 100% of the offspring are heterozygous (T t)

Phenotypic Percentage – 100% of the offspring are tall plants

Mendel’s First Cross Each parent only passes 1 of its alleles, so the tall parent always passed on a T and the short parent always passed on a t All the offspring had the genotype T t, because they received a T from the tall parent and a t from the short parent Because the tall allele is dominant, all the offspring were tall

Mendel’s Second Cross Mendel crossed two of the offspring from the first cross, which were both heterozygous T t x T t

Mendel crosses a heterozygous tall plant (Tt) with a heterozygous tall plant (Tt)

The first tall parent passes on either a tall allele (T)…

…or a short allele (t)

The second tall parent passes on either a tall allele (T)…

…or a short allele (t)

Possible offspring 1 – the first tall parent’s tall allele with the second tall parent’s tall allele (T T)

Possible offspring 2 – the first tall parent’s short allele with the second tall parent’s tall allele (T t)

Possible offspring 3 – the first tall parent’s tall allele with the second tall parent’s short allele (T t)

Possible offspring 1 – the first tall parent’s short allele with the second tall parent’s short allele (t t)

Genotypic Percentage – 25% of the offspring are homozygous dominant (T T)

Genotypic Percentage – 50% of the offspring are heterozygous (T t)

Genotypic Percentage – 25% of the offspring are homozygous recessive (t t)

Phenotypic Percentage – 75% of the offspring are tall plants

Phenotypic Percentage – 25% of the offspring are short plants

Mendel’s Second Cross Each parent only passes 1 of its alleles, but because each parent has 1 of each type of allele, they can pass either a T or a t Some of the offspring received two T alleles, some received one T and one t, and some received two t alleles The plants with genotype T T were tall, the plants with genotype T t were tall, and the plants with genotype t t were short