Chapter 14Mendel and the Gene Idea

Mendelian genetics

• Gregor Mendel – father of genetics

Austrian monk

pioneer in the field

gardener at monastery

Mendel’s Laws1) Law of Dominance and Recessiveness – when 2 different alleles are present, one masks, or covers up, another

2) Law of Segregation – alleles separate when gametes form

3) Law of Independent Assortment – one allele does not influence another ex: tall does not influence yellow

Probability

• Rule of Multiplication –

probability that 2 or more independent events

will occur simultaneously in some specific

combination

-take probability of each event and multiply together

-ex: both coins landing heads up

½ x ½ = ¼

or Pp x Pp = pp

Probability

• Rule of Addition – probability of an event that

can occur in 2 or more different ways

- add separate probabilities

- ex: heterozygote from Pp x Pp

¼ + ¼ = ½

Law of Incomplete Dominance –

(Intermediate inheritance)

- When 2 different alleles are present (heterozygous), an intermediate trait is expressed

ex: red flowers x white flowers = pink flowers

Multiple Alleles

- 3 or more alleles for 1 gene- ex: human blood groups

phenotype genotype antigens antibodies A IAIA or IAIa A anti–B B IBIB or IBIb B anti–A AB IAIB A or B O ii neither A anti–A

& nor B anti-B

Test cross

Pleitropy

• One gene = many effects

ex: sickle cell anemia

Penetrance

• Proportion of individuals who show expected phenotype from their genotype

- ex: neuroblastomas

Polygenic inheritance

• Many genes = one trait

-ex: human skin color

Pedigree analysis

Human Genetic Disorders

• Cystic fibrosis

• Tay-Sachs

• Sickle-cell anemia

• Huntington’s chorea

• Duchenne’s Muscular Dystrophy

• Down Syndrome

• Achondroplasia

Preventive Testing for genetic disorders

• usu. done when risk is high1) pedigree determination2) fetal testing: a) amniocentesis – 14th – 16th wk. of pregnancy; needle

inserted into uterus; 10 ml fluid extracted & karyotype done

b) chorionic villi sampling – sm. Amt. of fetal tissue is suctioned off from embryonic membrane villi

(chorion) which forms part of placenta, then karyotype (results in 24

hrs.) advantages: 24 hr results vs. several weeks ;

8-10 wks of pregnancy

Preventive

c) ultrasound – soundwaves

(noninvasive; no risk)

d) fetoscopy – tube with viewing scope

directly examines fetus

3) newborn screening –

PKU test

Chapter 15

The Chromosomal Basis of Inheritance

• Genes are located on chromosomes, the

structures that undergo segregation &

independent assortment

Thomas Hunt Morgan – 1st one to associate

specific genes with specific chromosomes

- studies with fruit flies, Drosophila melanogaster

1) grow rapidly

2) require small amt. of space

3) few chromosomes & these are large

• 1st to discover a sex-linked gene (white eyes)

X-linked

Sex-linked traits

• Carried on sex chromosomes

• May be X-linked or Y-linked

• No Y-linked found thus far

• ex: red-green color blindness in humans

Recombination

• In unlinked genes, when 2 organisms produce offspring, the end result could be:

parental types or recombinants (unlike either parent)

Frequency of recombination – if ½ have

different phenotype than the parent, we say there

is a 50% frequency of recombination (maximum)

Recombination frequency

Frequency of = # of recombinants

recombination total # of offspring x 100

Gene mapping

• map units - number assigned to show relative distance between genes on chromosomes

• recombination frequency = # of map units

ex: recombination frequency of 25% translates to 25 map units

Sex determination systemsa) X-Y system

-humans, mammals, some insects

-sperm (X or Y) determines sex

b) X-O system

-grasshoppers, crickets, roaches, some

insects (only 1 sex chromosome)

-female XX male XO

-sperm either contains X or O

Sex determination systemsc) Z-W system

-birds, some fishes, some insects (moths,

butterflies)

-Z & W used to avoid confusion with X-Y

-female ZW male ZZ

-egg determines sex

d) haplo-diploid system – most bees, ants

-no sex chromosome

-females develop from fertilized eggs (2n)

-males develop from unfertilized eggs (1n);

fatherless

Sex-linked traits

• carried on sex chromosomes

• may be X-linked or Y-linked

• example: red-green colorblindness, hemophilia

X-inactivation (in mammals)• fur color in calico cats determined by X

chromosome 1X orange fur, 1X black fur

• calico cats almost always female

• 2 X chromosomes inherited, but in embryonic

development, 1 is almost completely inactivated

(inactive X condenses to Barr body)

• selection of which X occurs ramdomly

• ex: in humans dev. of sweat glands (mosaicism)

heterozygous female have patches of normal skin & patches lacking sweat glands

Aneuploidies• abnormal number of chromosomes

• due to nondisjunction – failure of chromosomes

to separate in anaphase

1) trisomy – 2n+1

having 3 chromosomes in a pair

ex: Trisomy 21 (Down Syndrome)

2) monosomy – 2n-1

having only 1 chromosome in a pair

ex: Turner Syndrome

Polyploidy• having extra sets of chromosomes

• Triploidy (3n)

ex: diploid egg fertilized

• Quatraploidy (4n)

ex: 2n zygote may not divide

-fairly common in plants; almost nonexistent in animals (appear more normal than aneuploids)

Karyotype

Chromosomal mutations1) deletion – piece of chromosome is lost *most serious

2) duplication – extra piece of chromosome

3) inversion – piece of chromosome breaks off & reattaches in a different orientation

4) translocation – piece of chromosome breaks off & reattaches to a nonhomologous chromosome

Chromosome mutations

Genomic imprinting