GENETICS Gregor Mendel’s Discoveries Complex Inheritance Patterns of Inheritance

GENETICS

Gregor Mendel’s Discoveries

Complex Inheritance

Patterns of Inheritance

• Gregor Mendel: 1843 - Augustinian monastery.

• University of Vienna - 1851 to 1853

• Experimentation in causes of variation in plants.

• 1857 - breeding garden peas to study inheritance.

• Many varieties, distinct heritable features (characters) with different variants (traits).

• Mendel brought an experimental and quantitative approach to genetics.

Gregor Mendel

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings


• Hybrids

• The true-breeding parents - P generation

• hybrid offspring - F1 generation.

• Pollinate F1 hybrids - F2 generation.

• law of segregation

• law of independent assortment.

Law of segregation - the two alleles for a characteristics are

packaged into separate gametes


• 705 purple-flowered F2 plants• 224 white-flowered F2 • 3-1 ratio

• Dominant• Recessive


Table 14.1

Mendel’s hypothesis to explain the results.

1. Alternative version of genes (different alleles) account for variations in inherited characters.

• Different alleles vary somewhat in the sequence of nucleotides at the specific locus of a gene.

• The purple-flower allele and white-flower allele are two DNA variations at the flower-color locus.


Fig. 14.3

2. For each character, an organism inherits two alleles, one from each parent.• A diploid organism inherits one set of

chromosomes from each parent. - Each diploid organism has a pair of homologous chromosomes (two copies of each).

• These homologous loci may be identical, homozygous for that character (PP or pp)

• Alternatively, the two alleles may differ, heterozygous for that character (Pp).

• Ex. - a plant can inherit a purple-flower allele from one parent and a white-flower allele from the other.


3. If two alleles differ,

• The dominant allele, is fully expressed in the the organism’s appearance.

• The other, the recessive allele, has no noticeable effect on the organism’s appearance.

4. The two alleles for each character segregate (separate) during gamete production. (law of segregation)

• Distribution of homologous chromosomes to gametes in meiosis.


Review Questions

• Define P, F1, F2 generation.

• What does Mendel’s law of segregation state?

• What are the four parts of Mendel’s law of segregation?

• Define Homozygous and Heterozygous. What is another way to say homozygous?

• Define Dominant and Recessive

Punnett square

Fig. 14.4

•Predicts the outcome of a cross between two organisms.

•law of segregation - 3:1 ratio in F2 generation.

• F1 - two gametes:

• 1/2 purple-flower allele

• 1/2 white-flower allele

• gametes unite randomly

• four combinations

• Some vocab:

• description of traits - phenotype.

• The words that describe the character (Purple or White)

• genetic makeup - genotype.

• The letters we assign to the character (PP or Pp or pp)


• For flower color in peas, both PP and Pp plants have the same phenotype (purple) but different genotypes (homozygous and heterozygous).

• The only way to produce a white phenotype is to be homozygous recessive (pp) for the flower-color gene.


Fig. 14.5

Test Cross

• It is not possible to predict the genotype of an organism with a dominant phenotype.


• Dominance does not mean more common in a population.

• Polydactyly is due to an allele dominant to the recessive allele for five digits per appendage.

• The recessive allele is far more prevalent than the dominant allele in the population.

• 399 individuals out of 400 have five digits per appendage.


• Single character cross - monohybrid cross.

• Two different characters - dihybrid cross.

• Mendel crossed true-breeding plants that had yellow, round seeds (YYRR) with true-breeding plants that has green, wrinkled seeds (yyrr).

• The allele for yellow seeds (Y) is dominant to the allele for green seeds (y).

• The allele for round seeds (R) is dominant to the allele for wrinkled seeds (r).

Law of independent assortment


• two characters are not transmitted from parents to offspring as a package.• The Y and R alleles and y and r alleles do

not stay together.


• Two pairs of alleles segregate independently of each other.

• The presence of one specific allele for one trait has no impact on the presence of a specific allele for the second trait.


• Four distinct phenotypes in a 9:3:3:1 ratio.

• law of independent assortment - alleles must be on different chromosomes


Fig. 15.1

• Mendel used traits that were classified as complete dominance.• Each character (but one) is controlled by a

single gene.

• Each gene has only two alleles, one of which is completely dominant to the other.

Complete Dominance (recap)


Check in?

• Define Genotype and Phenotype

• What do we use a Punnett square for?

• Define Mendel’s Law of Independent Assortment

Beyond Mendel• We know that the relationship between

genotype and phenotype is rarely as simple as dominant and recessive.

• Mendel was never able to prove these…

• Incomplete dominance

• Codominance

• Multiple Alleles

• Polygenic inheritance

• Sex-linked traits

• Heterozygotes show a distinct phenotype, not seen in homozygotes.

• The dominant trait is notcompletely dominant overthe recessive.

• Result is a mixed phenotype.


Incomplete Dominance

Codominance

• Two alleles affect the phenotype in separate, distinguishable ways.

• There are two dominant alleles that both contribute to phenotype.

• MN blood groups - due to the presence of two specific molecules on the surface of red blood cells.

• (genotype MM) have one type of molecule on their red blood cells, (genotype NN) have the other type, MN (genotype MN) have both molecules.


Multiple Alleles (and codominance)• There are more than 2 alleles that

contribute to genotype.

• The ABO blood groups in humans are determined by three alleles, IA, IB, and i.

• Both the IA and IB alleles are DOMINANT to the i allele

• The IA and IB alleles are codominant to each other.



• Because each individual carries two alleles, there are six possible genotypes and four possible blood types.

RBC Surface Antigens

• Antigens are proteins found on the surface of a cell

Rh Factor with blood typing

• What about the + and – blood types?

• Rh factor is what determines the + or – blood typing.

• Alleles= Rh+ and Rh-

Stop and Review

• A woman with type A+, whose dad was O-, has a child with a man who is AB-. What are the potential blood types of their child

• A cat with a long tail is crossed with a cat who has a short tail. All of their offspring have medium length tails. Why? What alleles would you use?

Polygenic Inheritance• Additive effects of two

or more genes on a single phenotypic character.

• skin color in humans.

• An AABBCC individual is

dark and aabbcc is light.

• AaBbCc X AaBbCc

(intermediate skin shades)

produce offspring with a

range of shades.Fig. 14.12

• In addition to their role in determining sex, the sex chromosomes, especially the X chromosome, have genes for many characters.

• These traits typically follow the complete dominance principles discussed previously, but are linked to the X chromosome, as shown below.

• Men are affected more often by a sex linked trait.

Sex-linked traits


• Duchenne muscular dystrophy

• Absence of normal X-linked gene for a key muscle protein, called dystrophin.

• Progressive weakening of the muscles and loss of coordination.

• Hemophilia - absence of one or more clotting factors.

• Normal proteins slow and stop bleeding.

• Individuals with hemophilia have prolonged bleeding because a firm clot forms slowly.


Pedigree analysis reveals Mendelian patterns in human inheritance


heterozygotes are carriers - may transmit a recessive allele to their offspring, but do not suffer from the disease.

Recap and review?

• Distinguish between Incomplete dominance and Codominance.

• What are the three different alleles for human blood typing. How many geneotypes can be made with these alleles?

• Define Polygenic Inheritance.

• Why are men more susceptible to a sex-linked disease?

• What is a pedigree used for?

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GENETICS Gregor Mendel’s Discoveries Complex Inheritance Patterns of Inheritance