Genetics: the Study of Heredity

Look at these three girls What physical traits do you think they inherited from their parents? ______________

Heredity Living organisms receive traits from their parents. Traits are “characteristics that can be used to identify or describe an organism.” This passing on of traits from parents to their offspring is called “heredity.” Think of 3 physical traits you received from your parents: ____________, _____________, _____________.

How does this happen? Traits are passed from parents to their offspring. If 2 Dalmation dogs have puppies, what will the puppies look like? _________________________ Today we will look at how traits are passed from one generation to the next. What is a trait? http://learn.genetics.utah.edu/content/inheritance/traits/

Each person begins life as a tiny zygote… a single cell created from a female sex cell (egg) and a male sex cell (sperm). Each of these sex cells contains 23 chromosomes. Why? Human cells contain 46 chromosomes (23 pairs—half from the mother and half from the father) What is Heredity? http://learn.genetics.utah.edu/content/inheritance/intro/

Karyotype

What are chromosomes made of? D N A Proteins and deoxyribonucleic acid DNA carries information in a code. DNA looks like a twisted ladder. This shape is called a “double helix.”

Chromosomes carry genes Genes contain the instructions that determine what traits you will have. Hair color, eye color, and height are examples of traits determined by your genes. Genes are found in specific places on chromosomes.

How are traits (genes) passed on? You received a gene for eye color from your mother and a gene for eye color from your father. What determines which color gene will “show” in you? Back in the 1860’s, an Austrian monk named Gregor Mendel did a lot of research and discovered patterns of inheritance.

Mendel is known as the Father of Genetics Gregor Mendel did years and years of research experimenting with pea plants. Why pea plants?

Why Pea Plants??? Peas are: Easy to grow Reproduce quickly Produce many offspring Easy to “cross” (breed) through pollination Have several traits with 2 easy-to-see forms, such as purple or white flowers, yellow or green pods, smooth or wrinkled seeds.

Mendel observed that when he crossed a purebred short pea plant with another purebred short pea plant… The offspring were always short. When purebred plants were bred, their offspring looked just like themselves. Parents Offspring

Next, he crossed short with tall Parents The first generation of offspring were all tall BUT… When he crossed the those plants together, look what happened… A short one showed up in the next offspring generation! Offspring

How could a trait (shortness) that disappeared in the first generation show up in the second generation? Well, Mendel repeated this experiment many times, then came up with a hypothesis…

Mendel’s hypothesis: Mendel stated that if there is a PAIR of factors that determine a trait, then A tall offspring pea plant could have an tall factor that shows but still have a short factor that “hides.” The hidden shortness factor could get passed on to the next generation.

Let’s look at an example The top 2 seeds are the parents, or P generation The next row is their offspring, the F1 generation Look at F2 Which trait was hidden in F1? ________________ P F1 F2

Mendel’s Conclusions Each parent must contribute one factor to each offspring. These factors may be strong or weak. When a strong and weak factor are paired, only the strong one shows. A weak factor will be expressed ONLY if two weak factors are paired.

Dominant and Recessive Traits The stronger trait is called the DOMINANT trait. The weaker trait (the one that seems to disappear) is called the recessive trait. A professor of genetics at Cambridge University in England named Reginald Punnett developed a simple method for figuring out the probability that a trait will show up in offspring.

REGINALD PUNNETT (1875-1967) Punnett Square

A Punnett Square is divided into 4 boxes We will take a look at a pea plant cross-breeding experiment using a Punnett square

First, we will cross a purebred green pea plant (PP genotype) with a purebred yellow pea plant (pp genotype) PP x pp Note: each letter is called an “allele” P P Pp Pp p Pp Pp p

Describe the offspring from the PP x pp cross: What is their phenotype (what do they all LOOK like)? __________________ What is their genotype (pair of alleles)? ___ ___

Next, we cross two Pp genotype pea plants: Pp x Pp

Describe the offspring from the Pp x Pp cross: Do all the offspring look the same? In other words, do they all have the same phenotype? ______ What different genotypes (pairs of alleles) do the offspring have? ___ ___ , ___ ___ , ___ ___

Let’s say we want to cross two guinea pigs with different traits for fur color to see what their offspring will look like…

Must-have information: First, we need to know the genotype of each parent. Dominant alleles are symbolized by a capital letter (T for tan hair) Recessive alleles are symbolized by a lower-case letter (t for white hair) Each parent has 2 alleles that make up their genotype.

For example, Tan hair is a dominant trait in guinea pigs. A mother guinea pig has tan hair and her genotype is TT (homozygous) White hair is recessive. The father guinea pig is white, and white is recessive, so his genotype is tt (homozygous) Now we can set up our Punnett Square…

The cross is TT x tt The mother’s alleles are written on the top The father’s alleles are written along the side. Mom T T Tt Tt t Dad t Tt Tt What will the Tt offspring look like? _________________

Describe the next generation TT Tt t Tt tt