Mendel and the Gene Idea

Chapter 14

Father of Modern Genetics After failing to qualify as a biology teacher, the Austrian

monk Gregor Johann Mendel (1822-1884) began to research heredity in pea plants (1860’s).

He was the first to take a scientific, experimental approach and to quantify his data.

Particulate theory of heredity -- parents transmit separate inheritable factors (now called genes) to their offspring; replaced the blending hypothesis.

Why peas? : 1. There were many varieties. 2. Peas do not cross-fertilize, so he could have strict

control over mating. 3. Easy to grow many generations in a short amount of

time.

Mendel’s Work Mendel chose to study seven characters, each of

which occurred in two contrasting traits: 1. Flower color: purple(d) or white(r) 2. Flower position: axial(d) or terminal(r) 3. Seed color: yellow(d) or green(r) 4. Seed shape: round(d) or wrinkled(r) 5. Pod shape: inflated(d) or constricted(r) 6. Pod color: green(d) or yellow(r) 7. Stem length: tall(d) or dwarf(r)

Some vocab… True breeding -- Always producing offspring with the

same traits as the parents (self-pollination). P (parental) generation -- true-breeding parent plants. F1 (first filial) generation -- hybrid (cross-pollinated)

offspring of the P generation. F2 (second filial) generation – offspring of self-pollinated

F1 generation. Genotype – combination of genes in an organism. Phenotype – expressed traits in an organism. Allele – different forms of an gene for the same character.

Give peas a chance When Mendel crossed true-breeding plants with different characters,

the traits did not blend.

P generation: purple (PP) x white (pp) [homozygous dominant] [homozygous recessive]

F1 generation: all purple (Pp) x (Pp) [heterozygous]

F2 generation: purple(PP) purple(Pp) purple(Pp) white(pp)

Mendel found similar 3:1 ratios with the other 6 characters.

Mendel’s Principles 1. Organisms inherit two alleles for a trait, one from each

parent. 2. (Law of dominance) If the two alleles differ, one is

expressed (dominant) and the other is masked (recessive).

3. (Law of segregation) Two alleles for each character separate during gamete production; homologous chromosomes separate during meiosis.

• If different alleles are present in the parent, there is a 50% chance that a gamete will receive the dominant allele, and a 50% chance that it will receive the recessive allele.

Punnett Square

A chart used to predict probabilities of possible genetic outcomes.

Rule of multiplication -- probability that independent events will occur simultaneously is the product of their individual probabilities.

Monohybrid cross: a mating with reference to one character.

Mendel’s Principles (cont.)

4. (Law of independent assortment) -- Each allele pair segregates independently of other gene pairs during gamete formation; one gene does not influence the inheritance of a different gene.

Dihybrid cross: a mating with reference to two characters.

Test Cross

Crossing a parent with unknown genotype with a homozygous recessive parent.

Other patterns of inheritance (exceptions to Mendel’s rules)

1. Incomplete Dominance -- one allele is not completely dominant over the other; heterozygote’s phenotype is intermediate.

Snapdragon: red x white = pink SrSr SwSw SrSw

Incomplete dominance is not support for the blending theory of inheritance, because alleles maintain their traits.

Other patterns of inheritance (cont.)

2. Codominance -- both alleles in the heterozygote are fully expressed.

Human ABO blood groups Three alleles possible (multiple alleles), but

you only inherit two (one from each parent). IA IB i

phenotype/genotype rbc antigen/ serum antibody Type A IA IA or IAi Type B IB IB or IBi Type O ii Type AB IA IB

rh + RR or Rr rh - rr

A anti- B B anti- A None anti- A & B A & B none

rh + none None anti +

Other Patterns of Inheritance (cont.)

3. Pleiotropy -- The ability of a single gene to have multiple effects.

In some cats, a fur pigmentation gene also influences connections between cat's eyes and brain.

4. Epistasis – one gene alters the expression of a second gene.

The phenotypic ratio resulting from a dihybrid cross will deviate from the 9:3:3:1 Mendelian ratio.

In mice, the pigment production gene(C) is epistatic to the pigment color gene(B).

BB or Bb = black pigment; bb = brown pigment. CC or Cc = normal pigment; cc = no pigment Dihybrid cross will result in 9 black mice (CCBB, CCBb,

CcBB, CcBb), 3 brown mice (CCbb, Ccbb) and 4 white mice (ccBB, ccBb, ccbb).

Other Patterns of Inheritance (cont.)

5. Polygenic Inheritance – more than one gene determines a single character.

Produces quantitative characters that vary on a continuum.

Skin pigmentation is controlled by at least three separately inherited genes (A, B, and C).

AABBCC = very dark person; aabbcc = very light person.

AaBbCc = intermediate shade. Environmental factors, such as sun exposure,

could also affect the phenotype (nature vs. nuture).

Pedigrees Our understanding of Mendelian inheritance in humans is

based on the analysis of family pedigrees (a family tree that shows the inheritance pattern of a particular character among parents and children).

Squares symbolize males and circles represent females. A horizontal line connecting a male and female indicates a

mating; offspring are listed below in birth order, from left to right.

Shaded symbols indicate individuals showing the trait being traced.

Pedigrees can be used to predict probabilities.

Dominant Disorder

Autosomal Recessive Disorders Caused by defective recessive alleles (not on sex

chromosomes) that code for either a malfunctional protein or no protein at all.

Can be non-lethal (albinism) or lethal (cystic fibrosis). Disorders occur only in homozygotes (aa) who inherit one

recessive allele from each parent. Heterozygotes (Aa) are normal, but carriers (can transmit the

allele to offspring). Most people with recessive disorders are born to normal

parents, both of whom are carriers. Probability is 1/4 that a mating of two carriers (Aa x Aa) will

produce a homozygous recessive zygote. 2/3 chance that a normal child will be a carrier. Some of these disorders are more common in certain ethnic

groups: cystic fibrosis (caucasians),Tay-Sachs disease (central European Jews), and sickle-cell disease (African descent).

Autosomal Dominant Disorders

Caused by defective dominant alleles; only takes one copy to cause disorder.

Lethal dominant alleles are rarer in populations than lethal recessives.

In achondroplasia (dwarfism), AA is lethal in the fetus; Aa affects 1 in 10,000 people.

Huntington's disease (degenerative nerve disease) is caused by a late-acting lethal dominant allele; effects do not appear until 35 to 40 years of age, often after individuals have reproduced.

Children of an afflicted parent have a 50% chance of inheriting the dominant allele; a test can detect the Huntington's allele before disease symptoms appear.

Multifactorial Disorders Diseases that have both genetic and

environmental influences (heart disease, diabetes, cancer, alcoholism, and some forms of mental illness).

Traits are often polygenic and poorly understood.

Genetic Testing and Counseling

Carrier testing – identify genes in prospective parents and in embryos.

Fetal testing – blood tests, ultrasound, CVS (8-10 weeks), amniocentesis (14-16 weeks) and karyotyping.

Newborn screening – most states do routine blood test for phenylketonuria (PKU).