Genetics is the study of heredity : the passing of traits from parent to offspring Humans have 23 pairs of chromosomes. Segments of chromosomes that code for specific traits are called genes. Genes come in pairs.

Genes control traits. Chromosomes are made up of DNA. You received one set of chromosomes from your mother (23) and one set from your father (23) to make 23 pairs, or 46, chromosomes.

WHAT ARE ALLELES? Genes exist in alternative forms called alleles. Alleles are located on chromosome pairs. Body cells have two forms or ALLELES for a single gene (you got one from Mom and one from Dad) Many gene pairs follow Mendel’s law of dominance and recessiveness.

DOMINANT AND RECESSIVE A Dominant trait: will always be expressed and will “mask” a recessive trait A recessive trait can only be expressed if there are no dominant alleles present.

Example: Eyecolor—Brown color is dominant and blue is recessive. A person can have a brown allele and a blue allele but still have brown eyes because the brown allele is dominant and “hides” the blue allele.

Example: R= dominant r = recessive A capital letter is used to represent a dominant gene and a lower case letter is used to represent a recessive gene. Example: R= dominant r = recessive

Each organism is represented by TWO letters, one for each allele.

The phenotype of an organism is the physical appearance, or what you see. For instance, Blue or Brown Eyes.

The genotype is the actual genetic makeup. If both genes are the same, the individual is homozygous. For instance: BB or bb. If the two genes are different, the individual is heterozygous for that trait. For instance: Bb The alleles present in the organism are referred to as its genotype. For instance, BB, Bb, or bb.

Daughter has blue eyes (bb) Genetics by Brainpop Mom has blue eyes (bb) Dad has brown eyes (BB or Bb) Therefore, Dad must have the Bb genotype, because he must have given her the “b” allele. His “b” allele is masked by his “B”/dominant allele. Daughter has blue eyes (bb)

Genetic traits are not always dominant or recessive Genetic traits are not always dominant or recessive. Some traits will follow different patterns of inheritance. Incomplete dominance: Two different genes, when paired, blend together and produce a 3rd phenotype. Example: red snapdragons and white snapdragons produce pink snapdragons.

Incomplete dominance

Both alleles are expressed. Codominance Both alleles are expressed. EX: when a certain variety of black chicken is crossed with a white chicken, all of the offspring are CHECKERED. In other words, both feather colors are equally expressed.

Traits are found on the X chromosome. Sex-Linked Traits Traits are found on the X chromosome. The Y chromosome does not have a gene for the trait. Ex: Hemophilia – a recessive blood clotting disorder (hh) XhXh – female WITH hemophilia XHXh – female carrier XhY – male WITH hemophilia XHY – normal male

Blood type is also an example of codominance: Multiple alleles – traits controlled by more than two alleles in a population. Each individual has two genes for blood type, but 3 exist in our population. Blood type is also an example of codominance: The type A allele and the type B allele are codominant. Type O is recessive.

PROBABILITY A punnett square can be used to predict the possible genotypes and phenotypes of offspring. Before setting up your punnett square, you must first what genes can be in the parents’ gametes (sex cells)

During meiosis, the formation of sex cells, the gene pairs will separate and the gametes receive one of each. The gene for widow’s peak (W) is dominant. The gene for widow’s peak (w) is recessive. If one parent is heterozygous for widow’s peak & the other parent is homozygous for no widow’s peak, what are the possible genotypes of the offspring?

Heterozygous parent = Ww Homozygous parent = ww Put the possible gametes of one parent across the top of the Punnett square and the possible gametes of the other parent down the side. Set up the square for this cross and complete it.

Steps for using the Punnett square: Let’s say the parents are Bb and Bb (the same genotype). What would be their phenotype? B b So you would predict: ¼ offspring to be BB, or Brown Eyes 2/4 or 1/2 to be Bb, or Brown Eyes and ¼ to be bb or Blue eyes B BB Bb b Bb bb

So, if Miss Hemphill had children with her husband to be …what color eyes would they have? Mr. Ellinger Blue eyes (bb) Miss Hemphill Blue eyes (bb) b b bb b b All of their kids would have blue eyes!

Practice Problem #1 Predict the fur color of the offspring of a brown heterozygous hamsters and a white homozygous hamster. Brown is dominant. White is recessive. Use any letter you like.

Practice Problem #1 B b Bb bb b

Practice Problem #2 Predict the offspring of 2 white homozygous cows. Black is dominant over white. Use any letter you like.

Practice Problem #2 b b bb b

Practice Problem #3 Predict the offspring of 2 Grey Heterozygous rats. Gray is dominant over white, which is recessive. Use any letter you like.

Practice Problem #3 G g GG Gg gg G g

Practice Problem #4 Predict the pea shape of the offspring of one Heterozygous Round Pea plant and one homozygous wrinkled pea plant. Round is dominant over wrinkled. Use any letter you like.

Practice Problem #4 R r Rr rr r

Practice Problem #5 you don’t have to write this one down… What are the chances of having a boy or a girl? How would you solve this one?

Practice Problem #5 X Y XX XY X