• Gregor Mendel—the father of genetics
• Genetics—the scientific study of heredity
• Heredity—the passing of traits from parents to offspring
Mendel could learn a lot from peas because:
• Pea plants have many traits that exist in only 2 forms such as tall vs short plants, but no
• Peas produce a large number of offspring (kids) in each generation
HOW CAN PEA PLANTS REPRODUCE?
Normally pea plants self pollinate. This means that pollen from the stamen of a plant enters the pistil
of the same plant.
1. Self-pollination example: Pollen located on the stamen moves to pistil
Of the same plant
Mendel cross pollinated plants. This means that he took pollen from a plant and rubbed it on the pistil
of a different plant from which the stamens had been removed (so they couldn’t self pollinate).
2. Cross Pollination example: To visualize cross pollination, notice how pollen moves from the
stamen on plant A to the pistol on plant B.
• Purebred—always produces offspring with the same form of a trait as the parent.
• Ex. Purebred dogs. Like Cocker spaniels, golden retrievers, Poodles.
• The opposite is a hybrid. That would be like a pound puppy.
WHAT DID MENDEL DO, EXACTLY?
• In his first experiment Mendel crossed purebred tall plants (they only have tall genes) with
purebred short plants (that only have short genes).
• Filial (F1) generation—name given to the offspring of the first cross.
Results of F2
• Mendel allowed the F1 plants to self-pollinate.
• He found that the F2 plants were a mix of tall and short plants in a ration of ¾ tall to ¼ short.
You could also look at it like this:
F1 generation Why were all the kiddos tall here?
F2 generation Where did the little one come
¾ tall and ¼ short from? His parents were both tall?
TRAITS MENDEL STUDIED IN PEA PLANTS:
Mendel also studied 7 traits of pea plants: They were : Seed shape, seed color, seed coat color, pod
shape, flower position, flower color, and stem height.
DOMINENT AND RECESSIVE ALLELES
• Pairs—the factors that control each trait exist in pairs.
– Female parent—contributes one factor
– Male parent—contributes one factor
– Together these make a pair
• Genes—are the factors that control traits.
• Genes consist of pairs of alleles. One that comes from the mother parent and one that comes
from the father parent.
• Alleles—the different forms of a gene (such as tall or short, wrinkled or smooth).
• Dominant allele—one whose trait always shows up when the allele is present.
• Recessive allele—is masked (or covered up) when the dominant allele is present.
Recessive alleles only show up if a dominant allele is not present.
Practice: Write down the following answers. Will the baby show dominant or recessive traits?
• The only time the green box (or recessive allele) could show up is when a black box (or
dominant allele) was not present.
• This will lead us into punnett squares.
• So Mendel started out with 2 purebred plants.
One was tall and one was short.
• Capital T means “tall allele”, lowercase t means
• But, each of those two plants (tall one and short
one) has 2 alleles. They received one from their
mother and one from their father.
• Options are T(tall) or t(short)
• Tall plant was TT –purebred
• Short plant was tt--hybrid
• You take this information and put it in a Punnett
But what does it all mean?
The results represent the 4 possible offspring; which is the probability of children’s height.
Probability--The likelihood that a particular event will occur.
• Tossing a coin—landing head us is ?
• The larger the sample size (more tosses of a coin), the closer the actual results predicted by
• What about children. What is the likelihood that a woman would have a boy instead of a
Mendel used probability in genetics.
• Mendel was the first person to realize that probability can be used to predict the results of
• In other words he could use probability to “know” that all the offspring of the first generation
would be tall… without even seeing them.
In a cross between 2 hybrid plants (Tt x Tt), the probability of offspring having the dominant
characteristic is 75% (or 3 out of 4 or ¾) while the probability of the offspring having the recessive
characteristic is 25% (or 1 out of 4 or ¼)
In a cross between two purebred plants (TT x tt), the probability of the offspring having the dominant
characteristics is 100% (or 4 out of 4 which is the same as 4/4)
PHENOTYPE and GENOTYPE
• Phenotype—The physical appearance of the offspring. (ex. Tall or short)
• Genotype—The genetic makeup or allele combination of the offspring.
– Homozygous (purebred)—2 of the same alleles
• TT or tt
– Heterozygous (hybrid)—2 different alleles
• Tt or tT which are the same thing.
What are the genotypes present above ?___________________________________________________
• If there is one dominant (capital letter) trait, then that is the physical characteristic that shows
• Recessive traits only show up if there are no dominant traits present.
What are the phenotypes present above? __________________________________________________
• Alleles are neither dominant nor recessive. Neither allele is covered up. Both show up.
• Write them as capital letters with superscripts. (F or F = black or white feathers).
• This means that F F heterozygous chickens have both black and white feathers.
• Red hair and White hair are codominant in cattle. Heterozygous ( H H ) cattle have red hairs
and white hairs. They are called roan.
Yellow (yy) is recessive, Y is green, y is yellow
Green (YY) is dominant
What would be heterozygous/ hybrid?_____ How many children are green? _______
How many purebred children did you get?_____ How many children are yellow? ______
How many hybrid children did you get? _____
R=round, r=wrinkled : Y=yellow, y=green
Use the information above to draw and color in the large punnett square. You may talk in groups to try
REPRODUCTION IN CELLS, HEREDITY, AND GENES
Cells have a nucleus, Inside the nucleus of any cell are chromosomes,
Chromosomes are made up of DNA, Genes are sections of DNA.
Mitosis—cells have 46 chromosomes in their nucleus (2 of each kind)
Meiosis—“sex cells” have only ½ the number of chromosomes in their nucleus
1 sperm + 1 egg = zygote