1. N. Sassaman 8th grade Science 2011-2012
Genetics
N. Sassaman
8th grade Science

2. Chromosomes, Genes, and Alleles
Chromosomes are made up of many genes joined together like beads on a string. The chromosomes in a pair may have different alleles for some genes and the same allele for others.
DNA – the genetic material that carries information about an organism and is passed from parent to offspring.

3. Dominant vs. Recessive Alleles Gregor Mendel’s experiments
In all of Mendel’s crosses, only one form of the trait (dominant) appeared in the F1 generation. However, in the F2 generation, the “lost” form (recessive) of the trait always reappeared in about one fourth of the plants.
Trait – a characteristic that an organism can pass on to its offspring through its genes.
#1-5 on p. 12 packet

4. Dominant vs. Recessive Alleles
Letters are used to represent the alleles for a certain trait.
A capital letter is used for dominant trait (allele).
A lowercase letter is used for recessive trait (allele).
Examples: (see chart above)
T = Tall height G = green pod R = round seed Y = yellow seed
t = short height g = yellow pod r = wrinkled seed y = green seed
#6-13 on p. 12 packet

5. Homozygous vs. Heterozygous
REMEMBER: each gene for a trait has TWO possible variations called alleles. (one from mother, one from father)
THEREFORE: we assign TWO letters to an individual to represent which two alleles they have.
Examples: (see chart above)
T = Tall height G = green pod R = round seed Y = yellow seed
t = short height g = yellow pod r = wrinkled seed y = green seed
Height
TT = tall plant
Tt = tall plant
tt = short plant
Pod color
GG = green
Gg = green
gg = yellow
Seed shape
RR = round
Rr = round
rr = wrinkled
Seed color
YY = yellow
Yy = yellow
yy = green

6. Homozygous vs. Heterozygous
Homozygous (purebred) – having TWO OF THE SAME alleles for a trait. (homo = same)
Heterozygous (hybrid) – having TWO DIFFERENT alleles for a trait. (hetero = different)
Height
TT = tall plant
Tt = tall plant
tt = short plant
TT = homozygous tall (dominant)
tt = homozygous short (recessive)
Tt = heterozygous tall
Pod color
GG = green
Gg = green
gg = yellow
GG = homozygous green pod (dominant)
gg = homozygous yellow pod (recessive)
Gg = heterozygous green pod

7. Phenotype vs. Genotype pheno = physical (appearance)
An organism’s phenotype is its physical appearance, or visible traits, and an organism’s genotype is its genetic makeup, or allele combinations.
pheno = physical (appearance)
Ex/ tall or short
geno = genes (allele combo)
Ex/ TT, Tt, or tt
#1-3 on p. 16 packet; #1-3 on p. 18 packet

8. Codominance and Incomplete Dominance
Not all alleles for traits are expressed as a dominant/recessive relationship.
Codominance occurs when there are two variation of a trait (two alleles), but neither one is dominant over the other.
As a result, BOTH alleles are expressed if present in an individual.
FB = black feathers FW = white feathers
FBFB = black feathers
FWFW = white feathers
FBFW = BOTH black and white feathers

9. Codominance and Incomplete Dominance
Not all alleles for traits are expressed as a dominant/recessive relationship.
Incomplete dominance occurs when one allele for a trait is not completely dominant over the other allele.
R’R’
As a result, there is a ‘blending’ of the two specific traits.
RR’
R = red flowers R’ = white flowers
RR = red flowers
R’R’ = white flowers
RR’ = pink flowers
NOTE: the letters used to represent this type of inheritance pattern vary. Sometimes capital/lowercase are used, sometimes capital/capital prime, sometimes two capital letters. Always read problems carefully.

10. Probability and Punnett Squares
In a genetic cross, the allele that each parent will pass on to its offspring is based on probability, the likelihood that a particular event will occur.
Mendel was the first scientist to recognize that the principles of probability can be used to predict the results of genetic crosses.
For example: coat color in guinea pigs
Homozygous black (BB) X homozygous white (bb)
Geneticists use Punnett squares to show all the possible outcomes of a genetic cross and to determine the probability of a particular outcome.

11. Probability and Punnett Squares
How to set up a Punnett Square:
Draw a box with four (4) quadrants.
Take one parent and write its alleles ABOVE the two columns.
Take the other parent and write its alleles NEXT TO the two rows.
Fill in each of the boxes by writing the letters (alleles) that ‘match up’ in that box.
How to calculate probability:
From a Punnett square, all results will be expressed as ‘…out of 4’ or ‘-/4’.
For example: 2 out of 4 OR 2/4 of the offspring above have the genotype Tt.
1 out of 4 OR 1/4 of the offspring above have the genotype tt or TT.
A percent can then be calculated by dividing the fraction through and X 100 to get percent (%). 2/4 X = 50% ¼ X = 25%
# 4-5 on p. 16 packet; #5 on p. 19 packet

12. Homework!!
p. 17 in packet, #6-8
p. 19 in packet, #6-8

13. Class assignment: Page 13 in packet, #1-14
*Assume: B = black, b = white
Define key terms:
Multiple alleles
Sex-linked genes
carrier

14. p. 13 in packet 3. Black - 75%, White - 25%
4. Yes – Punnett square only gives chances of a certain outcome.
5. Black (Bb) and white (bb)
6. White is homozygous b/c two of the same alleles; black is heterozygous b/c two different alleles.
Black – 50%, White – 50% f a e g b c d

15. Multiple Alleles
Some human traits are controlled by a single gene that has more than two alleles.
Such a gene is said to have multiple alleles – three or more forms of a gene that code for a single trait.
Blood type is determined by a single gene with three alleles. This chart shows which combinations of alleles result in each blood type.
IA and IB are codominant to each other and both dominate over i (recessive).

16. Examples: hair color, eye color, skin color, height
Multiple Genes
Multiple genes that control a trait act together to produce a single trait with a large number of phenotypes.
Examples: hair color, eye color, skin color, height
Coat Color
Possible Genotypes
Black
BBEE BBEe
BbEE BbEe
Yellow
BBee Bbee
bbee
Chocolate
bbEE bbEe
Coat color in Labrador retrievers (general)
Gene #1 – Color (black or brown)
B – Black
b – brown
Gene #2 – Expression of color (yes or no)
E – expressed
e – not expressed

17. Sex-linked Genes Females have two X chromosomes - XX
The sex chromosomes carry genes that determine whether a person is male or female. They also carry genes that determine other traits.
Females have two X chromosomes - XX
Males have one X chromosome and one Y chromosome - XY
These are homologous chromosomes and separate during meiosis just like the other 22 pairs of homologous chromosome do!!

18. Red-green colorblindness
Sex-linked Genes
Some human traits occur more often in one gender than the other.
The genes for these traits are often carried on the sex chromosomes, most commonly on the X chromosome.
Genes on the X and Y chromosomes are often called sex-linked genes.
Because males have only one X chromosome, males are more likely than females to have a sex linked trait that is controlled by a recessive allele.
Red-green colorblindness
C – normal vision c – colorblind
Female: Male:
XCXC XCY
XCXc XcY
XcXc
Females, however, tend to be carriers of a recessive sex-linked trait (they have one recessive allele for a trait and one dominant allele)

19. Today’s assignment - p. 14 in packet!!Dd dd
Yes
50%
XCXc
XCY
XcXc
XcY
Yes – the father
50%
multiple alleles
Sex-linked traits
carrier
sex chromosomes

20. Pedigree - general
A pedigree is a chart or “family tree” that tracks which members of a family have a particular trait. (p. 118 in textbook)

21. Pedigree - example
The pedigree shows the inheritance of hemophilia, a sex-linked disorder in a family.

22. Genetic disorders
A genetic disorder is an abnormal condition that a person inherits through genes or chromosomes.
Examples:
Disorder
Description
Cause
Cystic fibrosis
Body produces abnormally thick mucus.
Recessive allele due to removal of three DNA bases.
Hemophilia
Blood clots slowly or not at all.
Recessive allele on X chromosome.
Down’s syndrome
Mental retardation and heart defects.
Extra copy of chromosome 21.