Transcript GENETICS CHAPTER 10

GENETICS
CHAPTER 10
GENETICS =
THE STUDY OF HEREDITY AND ITS VARIATION.
WE HAVE 46
CHROMOSOMES (23 PAIRS)
EACH CARRIES A
MULTITUDE OF DIFFERENT
TRAITS.
THE FATHER OF GENETICS:
GREGOR MENDEL
MENDEL’S WORK
•Gregor Mendel accomplished most of his work in the
MONASTERY in the small country of AUSTRIA.
•Working with garden peas, he worked with clear-cut
characteristics (TRAITS) so they could be easily
followed.
•Each of his chosen traits
occurred in alternate forms:
WRINKLED and SMOOTH
seeds, GREEN and YELLOW
color, tall and short, etc.
•Mendel mated pure (HOMOZYGOUS) individuals, one dominant
(TALL PEA PLANTS) the other recessive (SHORT PEA PLANTS) =
PARENTS. He called this the P1 generation.
•The offspring he received from
this mating were labeled the F1
generation.
P1
•Next he mated two F1 individuals
and a F2 generation was formed.
F1
F2
and
GENETIC VOCABULARY
 PHENOTYPE = The outward appearance of a
characteristic. Brown hair, blue eyes, tongue roller, etc.
 GENOTYPE = The genetic makeup of an organism. Tt, rr, etc.
 ALLELES = alternate forms of a gene, T and t, each are forms of
the same trait.
 HOMOZYGOUS = The alleles in a pair are identical. TT, hh, etc.
 HETEROZYGOUS = The alleles in a pair are idfferent. Tt, Hh, etc.
 HOMOZYGOUS DOMINANT = A genotype of two dominant
alleles. HH, TT.
 HOMOZYGOUS RECESSIVE = The genotype of two recessive
alleles. hh, tt.
 PUNNETT SQUARE = A chart used to solve genetic problems.
 MONOHYBRID CROSS = When a single trait is crossed.
Tt x TT, etc.
 DIHYBRID CROSS = When two traits are crossed at the
same time. TtRr x TTRR, etc.
 TESTCROSS = Mate a homozygous recessive individual with
an unknown genotype for that characteristic. Way to determine the
genotype of a homozygous individual. Example: BB or Bb x bb
 PEDIGREE = A family tree describing the occurrence of
heritable characters in parents and offspring across as many
generations as possible.
HOW ARE TRAITS PASSED FROM PARENT
TO OFFSPRING?
• Traits are transmitted by the CHROMOSOMES
which contain genes made of DNA.
HOW WAS THIS DISCOVERED?
Mendel studied traits with his pea plants. From the results of his
experiments Mendel made three conclusions (known as MENDELS
GENETIC PRINCIPLES)
•PRINCIPLE OF DOMINANCE – One gene in a pair may
prevent the other from being expressed.
• PRINCIPLE OF SEGREGATION – Each member of a gene
pair separates when the gametes form.
•PRINCIPLE OF INDEPENDENT ASSORTMENT – As each
gamete is formed, the genes are independently given to each
gamete.
LET’S PRACTICE SOME
MONOHYBRID CROSSES
A type of insect has either a red or black body, determined by
a pair of genes. The dominant allele, R, gives red, and the
recessive allele, r, gives black. Cross a heterozygous red
insect with a black insect. What are the phenotypic and
genotypic ratios of their offspring?
R
r
To solve the problem follow these steps:
1.
2.
3.
Choose the correct genotype letters for each
parent.
Put one letter above each box on the top of
the Punnett square to represent one parent.
Put one letter in each box on the side to
represent the other parent.
Multiply the top and bottom squares together
r
Rr
rr
r
Rr
rr
4. Finally determine the genotypic and phenotypic ratios.
GENOTYPE = DIFFERENT LETTER COMBINATIONS
2 Rr to 2 rr = 1Rr:1rr OR 50% Rr and 50% rr
PHENOTYPE = HOW THEY APPEAR TO US.
2 RED to 2 BLACK = 1 RED: 1 BLACK
Or 50% red and 50% black
PROBLEM #2 – Some people detect a distinctive taste from
the chemical phenylthiocarbamide in low concentrations while
others do not detect it at all. The ability to taste PTC is due to
the effect of a dominant gene. If a homozygous dominant PTC
taster mates with a nontaster, what % of their offspring will be
nontasters?
CHOOSE THE CORRECT LETTERS TO REPRESENT THE
PARENTS (Remember each parent gets two letters):
P = TASTER; p = nontaster
PARENTS = PP
x
pp
Correctly place the letters for each parent on one side of the
Punnett Square.
Multiply each box.
Count number of
individuals for a
particular phenotype –
nontaster – divide by 4 =
%
p
p
P
P
Pp
Pp
Pp
Pp
TEST CROSS
HOW DO YOU KNOW IF A DOMINANT TRAIT IS
HETEROZYGOUS OR HOMOZYGOUS DOMINANT?
By doing a TEST CROSS. Cross the DOMINANT individual
with a HOMOZYGOUS RECESSIVE individual.
SAMPLE PROBLEM:
White hair in rabbits is dominant over black hair? What is
the genotype of a white-haired rabbit, if it produces both
black and white haired offspring? The mate is black
haired.
What are the genotypes of the parents?
Ww
WW
White haired = ___________
or ___________
ww
Black haired = ___________
ww
Ww
What are the genotypes of the offspring: ______ and _______
w
W
w
Ww
ww
W
W
w
All Ww
w
Ww
ww
w
DIHYBRID CROSSES
The type of offspring can be determined for two different traits
at the ___________________ time by doing a dihybrid cross.
Sample Problem:
Widow’s peak is a recessive trait. Tongue rolling is a dominant
trait. A person that is heterozygous for tongue rolling and
heterozygous for no widow’s peak mates with an individual that
has a widow’s peak and cannot roll his tongue. What percent of
their offspring will be heterozygous for both traits? What percent
of the offspring will be widow peaked, tongue rollers?
•Determine the genotype of each parent. Since there are two traits, how
4
many letters should you have for each parent? _________
wwtt
WwTt
Genotypes of parents: _______________
x _______________
•Determine the different possibilities of gametes that each parent can
pass on.
Parent 1 gamete possibilities:
WT, Wt, wT, wt
wt
Parent 2 gamete possibilities:
•Put all the different gamete possibilities for Parent 1 in the top boxes of
the Punnett Square and all the different gamete possibilities for Parent 2
in the side boxes.
•Multiply each of the boxes, enter the offspring letters in each box.
•Count the number of offspring that are heterozygous for each trait.
Divide by the total number of boxes in the Punnett Square. This equals
your percent.
•Look at the genotypes and determine which are recessive for both traits.
Take this number and divide by the total number of boxes in the Punnet
Square. This equals the percent for question #2.
WT
wt
WwTt
Wt
wT
wt
Wwtt
wwTt
wwtt
Number of Heterozygous for both
traits - _________
1
Total number of Punnett boxes –
____________
4
% Offspring heterozygous
for both traits = 25% (1/4)
What % will be widowpeaked, tongue rollers?
¼ = 25%