Chapter 6

Heredity

Section 1: Mendel and His Peas Humans share common

characteristics i.e. feet, eyes, hands, etc.

But what makes you different than your fellow classmates? Heredity – the passing of traits from

parents to offspring How heredity works was the topic

of Gregor Mendel’s experiments

Who Was Gregor Mendel Mendel was born in 1822 in Austria He grew up on a farm and learned

about cultivating flowers After attending college, Mendel entered

a monastery, where he worked in the garden

He used the monasteries plants (garden peas) to study how traits where passed on from parents to offspring

Unraveling the Mystery Mendel’s choice of garden peas was a

good one for several reasons 1) they grow quickly 2) they are self-pollinating 3) they come in many varieties

Self-pollinating plant – a plant that contains both male and female reproductive structures

Peas Be My Ponder To keep things simple Mendel

decided to study only one trait at a time i.e. height or flower color

Pea plants have two forms for each characteristic i.e. tall vs. short plants or purple vs.

white flowers

True –Breeding Plants Mendel was careful to use plants that

were true-breeding True-breeding plant – plants that self-

pollinate and always produce off-spring that have the same trait as the parent plant

i.e. tall parent plant produces 100% tall offspring Mendel decided to “cross” two plants

with different forms of the same trait i.e. tall plant and short plant

He used a method called “cross-pollination”

Mendel’s First Experiment In his first experiment Mendel studied seven

different characteristics (we will focus on one – flower color)

Mendel crossed a true-breeding purple flowered plant with a true breeding white flowered plant (parental generation)

The offspring are known as the first generation(f1) All of the offspring had purple flowers!

Mendel got similar results from all his different crosses

What happened to the white flowers? One trait always appeared in the f1 generation,

while the other seemed to vanish Dominant trait – the trait that appeared Recessive trait – the trait that seemed to vanish

Mendel’s Second Experiment

Mendel allowed the f1 generation to self pollinate

Do you know what happened? The recessive trait (white flower)

showed up again The results were 75% purple to 25%

white This is a 3:1 ratio

A Brilliant Idea Mendel realized that his results could be

explained only if each plant had two sets of instructions for each characteristic Each parent could only donate one set of

instructions to the offspring Genes – the set of instructions What do these instructions control? Alleles – two genes that govern the

same characteristic

The Proof Is in the Punnett Square Punnett Square – a tool used to

visualize all the possible combinations of alleles

Lets do some examples on the board

Genotype – The inherited combination of alleles

Phenotype – The organism’s outward appearance

What Are the Chances?

It is important to understand that offspring are equally likely to inherit either parentsallele

Even though there seems to be a 50/50 chance of inheriting either allele, it is more similar to a coin toss and is completely randomBecause of this the laws of probability must be considered Probability - is the mathematical chancethat an event will occur

Genotype Probability

The same method is used to calculate the probability that an offspring will inherit acertain genotype

For example in Mendel’s 2nd experiment what is the probability that an offspring will inherit two lowercase p’s (pp) and be white?(1/2 x 1/2 = 1/4 = 25%)

Chapter 6 Quiz 1

1)_________ is the passing of traits from parents to offspring2) _________ is a plant that contains both male and female reproductive structures3) _________ the name Mendel gave to the trait that seemed to disappear in his 1st experiment4) _________ are two forms of the same gene (one on moms chromosome and one on Dads)5) __________ is a tool used to visualize all possible combinations of inherited genesBonus) __________ carry the genes that determine whether and organism is male or female

Section 2: Meiosis

Two Kinds of Reproduction Asexual Reproduction – only one parent is

needed 1) the internal structures are copied (mitosis) 2) the cell then divides producing two identical cells

Most single-celled organisms reproduce this way as well as most of the cells in your body

Sexual Reproduction – two parent cells join together to form a new individual

Sex cells – the parent cells (sperm/eggs) contain half the normal amount of chromosomes

Homologous chromosomes – Identical chromosome pairs

i.e. Humans have 23 pairs of chromosomes (46 total), 23 from mom and 23 from dad

Why do sex cells have ½ the amount of chromosomes?

Meiosis to the Rescue Walter Sutton discovered that genes are

located on chromosomes Sex cells are made during meiosis

Meiosis – produces new cells with ½ the usual number of chromosomes

During meiosis the cells are copied once, but go through two rounds of divisions producing sperm and eggs with ½ the normal amount of chromosomes

Male or Female

Sex chromosomes – Chromosomes that carry genes that determine whether an offspring will be male or female In humans females have two X

chromosomes, while males have one X and one Y

Chapter 6 Quiz 2

1.________ is the processes that produces new cells with half the number of chromosomes2.________ are pairs of chromosomes that contain the similar genes for the same traits 3.________ is the phase of meiosis where crossing over occurs4.________ is the scientist who discovered thatgenes where located on chromosomesBonus) ________ are flowers that always have offspring identical to themselves