Transcript Chapter 8 Mendel & Heredity

Chapter 8
Mendel & Heredity
I. Terms
A. Heredity- the passing on of traits (color/shape
of eyes, texture of hair) from parent to offspring
B. Genetics- the study of heredity
C. Genes- pieces of DNA that code for a certain
trait (Mid-digital hair)
D. Alleles- alternative versions for each gene
(ex. hairy mid-digits, non hairy mid-digits)
1) Letters are used to represent alleles (Capital
letters represent dominant trait, lower case
represent recessive trait)
Terms continued
E. Homozygous- 2 alleles in an individual
are the same (ex. HH, hh), pure bred
F. Heterozygous- alleles are different (ex.
Hh), hybrid
G. Genotype- the set of alleles that an
individual has (ex. HH, Hh, hh)
H. Phenotype- the physical appearance of
a trait (ex. Hairy mid-digits, non hairy mid
digits)
II. Gregor Mendel
A.
Gregor Mendel (1800’s) - Father of
Genetics p.161
B.
http://science.discovery.com/videos/100-greatest-discoveriesshorts-genetics.html
Mendel
B. Studied 7 traits in garden peas (grows
quickly & produces many offspring)
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***Traits- pod color, flower color,
wrinkled, height, seed shape, etc.
-Self-pollination produced the same traits
every time
Ex. Purple x Purple = All Purple flowers
White x White = All White
• Crossed 2 plants with contrasting traits
(P1 x P1 = F1)
– Ex. Pure white x Pure purple = All purple plants
F1 (filial) generation
• 2nd cross between plants of F1 generation
yielded the F2 generation
– F1(purple) x F1 (purple) = F2 (75% purple & 25%
white)
• *(Before Mendel, people thought offspring
were a blend of their parents
Ex. Tall x Short = Medium height)
C. Mendelian theory of heredity
1. For each trait, an individual has two copies of
the gene—one from each parent
2. There are alternative versions of genes
(alleles)
3. The allele that is displayed is called dominant;
the allele that is present in the organism but
has no effect on its appearance is called
recessive
1.Law of Segregation= the two alleles for the
trait separate when gametes are
formed. Gametes carry only one allele
for each inherited trait
2. Law of independent assortment= the
alleles of different genes separate
independently of one another when gametes
are formed.
III.Punnett square
A. Tool used by scientist to predict the
outcome of a genetic cross
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Monohybrid sample problem:
In roses, red is dominant over white.
Use R for the red gene.
Use r for the white gene.
Cross two heterozygous red roses,
Rr X Rr
• From the Punnett Square,
– describe the phenotype of the
offspring.
• 3/4 Red
• 1/4 White
– describe the genotype of the
offspring.
• 1/4 RR
• 1/4 rr
• 2/4 (or ½) Rr
• </TD
IV. Beyond Complete Dominance
A. Incomplete dominance- when neither allele
is dominant over the other; results in
blending the heterozygous genotype (Rr)
Examples:
Incomplete Dominance:
#1. One trait in humans that is displayed by
incomplete dominance is hair texture. The
heterozygous genotype (Hh) is expressed as
wavy. If mom has curly hair, which is dominant
to straight hair, and dad has wavy hair then what
is the probability of them having an offspring that
has curly hair?
Can they have an offspring that has
straight hair?
B. Co-dominance- both alleles are expressed in
the heterozygote equally
Ex. Multiple alleles
inheritance (blood type)
Example
Co-dominance
#2. Roan cattle are a result of codominance. List the possible
phenotypes of the offspring that would
result from the crossing of 2 roan
cattle.
C. Sex-linked inheritance- traits carried on
the X chromosome
ex. color blindness (red/green),
hemophilia (blood clotting disorder)
Punnett Square Examples:
XX=female
XY=male
More Examples of Sex-Linked Recessive
Disorders
• Red/Green Colorblindness – Difficulty perceiving differences
between colors (red or green, blue or yellow).
• Hemophilia – Absence of one or more proteins necessary for
normal blood clotting.
• Deafness
• Cataracts – opacity in the lens that can lead to blindness
• Night blindness – (Nyctalopia) rods do not work so that can not see
in the dark
• Glaucoma – pressure in the eye that can lead to optic nerve
damage and blindness
• Duchenne Muscular Dystrophy – progressive weakness and
degeneration of skeletal muscles that control movement due to
absence of dystrophin (protein that maintains muscle integrity).
Mainly in boys, onset 3-5 yrs, by 12 years can’t walk, and later
needs respirator.
Example
Sex-linked disorders:
#3. Hemophilia is a blood clotting disorder
that is sex-linked recessive disorder (found
on the X chromosome). Demonstrate a
cross between a mom who is a carrier for
the disorder and a dad who does not have
hemophilia.
a) What percent of the offspring will have hemophilia?
b) What percent of the females are carriers?
c) What percent of the males have hemophilia?
Example
Sex Linked Inheritance & Co-dominance:
#4. Coat color in cats is a co-dominant trait
and is also located on the X chromosome.
Cats can be black, yellow or calico. A
calico cat has black and yellow splotches.
In order to be calico, the cat must have an
allele for the black color and an allele for
the yellow color.
#4. A female calico cat is crossed with a
male black cat.
a) What percent of the offspring are
calico?
b) What percent of the females are calico?