DNA and Genetics Section 12.1-12.2

Genetics The study of how characteristics or traits are passed from parents to offspring. To understand how you ended up with the traits you did, first let’s find out what you got from your parents….

“WHO AM I” Activity

So how did you get these characteristics? YOUR PARENTS But how???

DNA and Chromosomes Your body is made of 10 trillion cells Each chromosome consists of a double helix DNA strand DNA is divided into segments called genes, which determine your inherited traits Each cell contains a nucleus with chromosomes

How does it all fit together?

DNA – All of the instructions for your cells are in your DNA Double – helix shape (twisted ladder) A pair of nitrogen bases make up each rung One DNA molecule contains 3 billion base pairs Different genes are made up of different arrangements of base pairs The sequence of base pairs determines the traits you inherit

Chromosomes Human body cells contain 46 chromosomes arranged in 23 pairs that hold all the genetic information. One chromosome of each pair comes from the mother and the other from the father Is this individual a male or female? How can you tell?

Did you know…. The DNA of all people on Earth is 99.9% identical. It is the 0.1% difference that makes each one of us unique

Heredity The passing of characteristics from parents to offspring Gregor Mendel, an Austrian monk, experimented with pea plants to show that traits didn’t happen by accident, rather they were passed on from generation to generation. You will copy his experiments in your computer lab tomorrow!

Mendel’s experiments t t T T Mendel looked at the results of crosses between two plants He referred to parent plants as purebreds They have identical genes for each trait TT (tall) versus tt (short)

Mendel’s experiments When Mendel crossed a pure tall plant (TT) with a pure short plant (ss) the offspring were always tall He called tallness a dominant trait because it seemed to dominate the short trait He called shortness a recessive trait because it seemed to disappear Offspring of this cross are called hybrids. - Hybrids carry one gene of each kind T t T t T t T t

Mendel’s experiments When Mendel crossed hybrid tall plants with hybrid tall plants, he was surprised to discover that sometimes shortness appeared in the offspring From these observations, Mendel concluded that tallness must be controlled by factors inherited from both parents

So how do you explain Mendel’s results? Background: T – capital letters represent dominant genes (in this case tallness) t – lowercase letters represent recessive genes (in this case shortness) Genotype: letter code for the combination of genes that an organism has for each trait

Punnett Squares So if we know the genotype of a parent plant, can we predict what the offspring will be? Absolutely, but we need to use a … Punnett Square – tool used by geneticists to predict the probability of offspring inheriting certain traits T T t This is why all of the first generation plants were tall T t T t t T t T t

Mendel’s experiments T T t t T t T t T t T t

Draw the Punnett Square for when Mendel mated two hybrids (Tt) So 25% will be TT, 50% will be Tt and 25% will be tt

Mendel’s experiments T T t t T t T t T t T t T T T t T t t t

Now it’s your turn… b b Bb Bb B b bb bb A female who has brown hair and genotype Bb mates with a male who has blonde hair and a genotype bb. Complete a Punnett square to discover the probability of their children having blonde or brown hair. b b Bb Bb B b bb bb

Now it’s your turn… Females have an XX genotype and males have an XY genotype. Complete a Punnett square to discover the chances of giving birth to a male or female child. X X XX XX X Y XY XY

Pedigrees Family tree; diagram used by geneticists to show the history of a trait passed from generation to generation Symbols used: Child 2 in Generation 2 Oldest children always listed first (farthest to left)

Now it’s your turn to interpret a pedigree…