Genetics
Heredity
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Genes code for specific proteins which determine the traits (characteristics) of the individual organism.
***Gene (DNA)  RNA  Protein  Traits
Phenotype vs Genotype
A phenotype
Examples: tongue roller, blood type, hitchhiker’s thumb
A genotype describes which genes a person has, whether they are expressed or not, and is represented by a pair of letters for each gene.
Examples:
Contrast…
In your own words, explain the difference between the phenotype and genotype of an organism.
Alleles
Many traits typically have different possible versions (examples: angora fur or short fur; regular thumb or hitchhikers’ thumb; blood type A, B, AB, O). The different versions of these genes are called alleles.
1. Each organism gets one allele (gene version) from their mother and one from their father.
2. i. Dominant alleles always express their trait, no matter what the other trait from the other parent is. Dominant alleles are represented using a capital letter. (RR, WW)
**Homozygous Dominant =
** Heterozygous =
Ii. Recessive alleles do not express their trait when the partner allele is dominant; they are expressed only when both alleles are recessive. Recessive alleles are represented using a lowercase letter. (rr, ww)
** Homozygous Recessive = two small letters
Analyze:
Blonde hair color in humans is recessive to dark hair color.
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It is expressed only if the person inherited the blonde allele from both parents; otherwise the person will express dark hair. Suppose that person A has natural blonde hair, and her mother is also naturally blonde, but her father has brown hair. How is this possible? (Consider the phenotypes and possible genotypes of each parent.)
Traditional Inheritance – Monohybrid Crosses
A monohybrid cross shows the probable outcomes (results) of one particular trait in the offspring, using the parental generation’s alleles.
Example: Breeding two tall heterozygous (Tt) pea plants with each other may result in tall homozygous (TT), tall heterozygous (Tt), or short (tt) plants.
Tt × Tt = TT or Tt or tt
Punnett Squares
Punnett squares are used to organize the ______________ combinations of alleles when doing crosses for traits that follow traditional inheritance rules.
Since each parent has two alleles from which to choose one to transmit to the offspring, a simple Punnett square is a
2×2 table (two columns and two rows).
T is the dominant trait (T = tall); t is the recessive trait (t = short)
APPLY
Suppose you cross two pea plants. Consider the alleles P for yellow peas and p for green peas. If the male plant’s genotype is Pp and the female’s genotype is pp: a) What are the possible genotypes of the offspring? b) What are the phenotypes of each parent and of the possible offspring?
(Show probabilities as fractions and percentages)
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Non-Traditional Inheritance
Some alleles are not dominant or recessive. This is called non-traditional inheritance. We will learn about three types.
Incomplete Dominance
A.
Incomplete dominance:
Example: a mouse that has a genotype of BW (B = black, W = white) is not black, but grey.
Codominance
B. Codominance:
Example: a hen that has the genotype BW (B = black feathers, W= white feathers) does not have black feathers, but feathers speckled black and white.
Polygenic Traits
C.
Polygenic traits:
Example: body height potential (not considering nutrition); people come in many sizes, from short to tall and everything in between.
SORT: Carefully examine each set of traits below and decide whether the trait is an example of: traditional inheritance
(T), codominance (C), incomplete dominance (I), or polygenic trait (P).
Flower Color
Coat Pattern
Height of a Sequoia Tree
Face Shape
Eye Size
Sheep Eye Color
Human Skin Color
Fur Color: Black+White=Gray
Fur Color: Orange+White= Orange & White
APPLY
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Use the information about mouse fur color in the example for incomplete dominance.
Suppose you cross two mice. If the mother is BB and the father is heterozygous, what are the genotypic and phenotypic ratios (as fractions and as percentages)? Draw a Punnett square to support your answer.
Complex Inheritance
Multiple Alleles:
Example: Human blood types have three alleles: IA, IB, and i.
These alleles can also be written as _____________________
Rules for pairing blood alleles:
A (IA) and B (IB) are dominant alleles and O (i) is a recessive allele.
A and B are codominant to each other
As a result, there can be 4 different phenotypes of human blood types:
A, AB, B and O
Genotype IA IA = Phenotype A
Genotype IA IB = Phenotype AB
Genotype IB IB = Phenotype B
Genotype ii = Phenotype O
Genotype IA i = Phenotype A
Genotype IB i = Phenotype B
The only way to display blood type O is to have two recessive alleles (ii).
APPLY
APPLY: 1. Dad is homozygous for blood type A and mom is homozygous for blood type O.
Dad’s Genotype?:
Mom’s Genotype?:
Set up and fill in the Punnett Square:
Mom: Dad:
Can a couple with type AB & O have a child with O blood?
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Sex-Linked Traits
Some traits, such as colorblindness, are linked to the sex chromosomes. The genes for these traits are found on the X chromosome; the gene is missing in the Y chromosome because it’s shorter. Recessive sex-linked traits are usually expressed more in males because they only have one X-chromosome and therefore cannot be dominated by a second allele.
Apply
A couple is having their first child this summer. The father is colorblind, while the mother is not, but she is a carrier. If they have a boy, what is the likelihood that he will be colorblind as well? What about if they have a girl? Use the Punnett square to find your answer.
Restate
RESTATE: Read the paragraph for “Sex-linked Traits” and restate what it says regarding why is a sex-linked recessive trait more prevalent in men than women.
Pedigrees
Genetic traits can be traced through families using pedigrees, or genetic family trees
A.
B. Males are shown as squares and females are shown as circles.
C. Parents are shown connected to each other with a horizontal line between them.
D.
E. When tracing specific traits, dominant traits are usually shown as unfilled (white) and recessive traits are shown as filled (dark).
Apply
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Dihybrid Crosses
When we study the inheritance of two traits at the same time we call it a dihybrid cross. Punnett squares can also be used to predict the probable outcomes of these crosses. To do so, we follow the rules listed below:
A.
B. The probability for each possible offspring is calculated, just as with a monohybrid cross.
Example: In a species of fictitious mice, black fur (B) is dominant over brown fur (b). Most mice run (R); however, a recessive allele for this gene (r) causes the mice to hip-hop instead. Suppose that you cross two mice with the following genotypes:
Parents: BbRr X bbRr
First, we need to know which gamete genotypes we can use to set up the Punnett square. This is not as simple as B or b; these are dihybrid crosses!
From meiosis we know that gametes must contain only one allele for each gene, and they can be sorted into alternative combinations if the genes are on separate chromosomes. To determine the possible gamete genotypes in dihybrid crosses, each gamete must show one allele for each of the two genes.
By a similar process, the possible gamete genotypes from the bbRr parent can be determined to be bR and br.
There are many snake breeders that try to breed for unusual colors and patterns. Shown below is a cross between two heterozygous green striped snakes (GgSs). These snakes also have the recessive genes for yellow color (g) and for solid patterns (s).
Parents: GgSs X GgSs
Possible gametes:
Punnett Square
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