Genetics

N. Sassaman8th grade Science

2011-2012

Chromosomes, Genes, and AllelesChromosomes 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.

Dominant vs. Recessive AllelesGregor 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

Dominant vs. Recessive AllelesLetters 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 seedt = short height g = yellow pod r = wrinkled seed y = green seed

#6-13 on p. 12 packet

Homozygous vs. HeterozygousREMEMBER: 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 seedt = short height g = yellow pod r = wrinkled seed y = green seed

HeightTT = tall plantTt = tall plant

tt = short plant

Pod colorGG = greenGg = greengg = yellow

Seed shapeRR = roundRr = round

rr = wrinkled

Seed colorYY = yellowYy = yellowyy = green

Homozygous vs. HeterozygousHomozygous (purebred) – having TWO OF THE SAME alleles for a trait. (homo = same)

Heterozygous (hybrid) – having TWO DIFFERENT alleles for a trait. (hetero = different)

HeightTT = tall plantTt = tall plant

tt = short plant

Pod colorGG = greenGg = greengg = yellow

TT = homozygous tall (dominant)

tt = homozygous short (recessive)

Tt = heterozygous tall

GG = homozygous green pod (dominant)

gg = homozygous yellow pod (recessive)

Gg = heterozygous green pod

Phenotype vs. GenotypeAn 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

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 feathersFWFW = white feathers

FBFW = BOTH black and white feathers

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.

As a result, there is a ‘blending’ of the two specific traits.

R = red flowers R’ = white flowers

RR = red flowersR’R’ = white flowersRR’ = 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.

R’R’

RR’

Probability and Punnett SquaresIn 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.

Probability and Punnett SquaresHow to set up a Punnett Square:

1. Draw a box with four (4) quadrants.2. Take one parent and write its alleles ABOVE the

two columns.3. Take the other parent and write its alleles NEXT

TO the two rows.4. Fill in each of the boxes by writing the letters

(alleles) that ‘match up’ in that box.

How to calculate probability:

1. 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.

2. A percent can then be calculated by dividing the fraction through and X 100 to get percent (%). 2/4 X 100 = 50% ¼ X 100 = 25%# 4-5 on p. 16 packet; #5 on p. 19 packet

Homework!!

• p. 17 in packet, #6-8• p. 19 in packet, #6-8

Class assignment:

• Page 13 in packet, #1-14*Assume: B = black, b = white

• Define key terms:– Multiple alleles– Sex-linked genes– carrier

p. 13 in packet3. 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.7. Black – 50%, White – 50%8. f 9. a 10. e 11. g 12. b 13. c 14. d

Multiple Alleles

Blood type is determined by a single gene with three alleles. This chart shows which combinations of alleles

result in each blood type.

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.

IA and IB are codominant to each other and both dominate over i (recessive).

http://www.zerobio.com/videos/blood_type.html

Multiple GenesMultiple 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 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

Coat Color Possible Genotypes

BlackBBEE BBEe BbEE BbEe

YellowBBee Bbee

bbee

ChocolatebbEE bbEe

Sex-linked GenesThe 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!!

Sex-linked GenesSome 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.

Females, however, tend to be carriers of a recessive sex-linked trait (they have one recessive allele for a trait and one dominant allele)

Red-green colorblindness

C – normal vision c – colorblind

Female: Male:XCXC XCYXCXc XcY

XcXc

Today’s assignment - p. 14 in packet!!

1. Dd2. dd3. Dd4. dd5. Yes 6. 50%

7. XCXc

8. XCY9. XcXc

10. XcY11. Yes – the father12. 50%13. 50%

14. multiple alleles15. Sex-linked traits16. carrier17. sex chromosomes

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

Pedigree - example

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

http://www.dnalc.org/view/16315-Animation-13-Mendelian-laws-apply-to-human-beings-.html

Genetic disordersA 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.