Patterns of Inheritance

Chromosomes

• Karyotype: Entire set of chromosomes from a single cell• Chromosomes. Each double DNA helix is packaged into a chromosome.• Every species has a particular number of chromosomes• Hereditable information exists in discrete units called genes• A gene is the specific portion of chromosome that has the complete

information to synthesize … a specific protein

Gregor Mendel• Before settling down as a monk in the monastery of

St. Thomas in Brünn.

• Gregor Mendel tried his hand at several pursuits, including health care and teaching.

• He studied botany and mathematics among other subjects. This training proved crucial to his later experiments, which were the foundation for the modern science of genetics.

• Although Mendel worked without knowledge of genes or chromosomes, we can more easily follow his experiments.

Genetic concepts• A gene´s specific physical location on a chromosome is called locus

(plural, loci).

• Homologous chromosomes carry the same genes, located at the same loci.

• Alternate forms of the gene, is called alleles.

• If both homologous chromosomes in an organism have the same allele are a given gene locus, the organism is said to be homozygous at the gene locus

• If two homologous chromosomes have different alleles at a given locus, the organism is heterozygous.

The M locus contains the M gene, which is involved in determining leaf

color. Both chomosomes carry the same allele of the M gene. This

tomato plant is homozygous for the M gene.

The D locus contains the D gene, which is involved in determining plant height. Both chromosomes carry the

same allele of the D gene.This tomato plant is homozygous for the D gene.

The Bk locus contains the Bk gene, which is involved in determinig fruit shape. Each chromosome carries a different allele of the Bk gene. This

tomato plant is heterozygous for the Bk gene.

Pair of homologous chromosomes

Mendel´s results, allow us to develop a five-part Model:

1. Each trait is determined by pairs of discrete physical units (alleles), which we now call genes. Each individual has two genes for a given trait.

2. Pairs of genes on homologous chromosomes separate from each other during gamete formation, so each gamete receives only one allele of an organism´s pair of genes. This conclusion is known as Mendel´s law of segregation. Each gamete receives only one of each parent´s pair of genes for each trait.

3. Which member of a pair of genes becomes included in a gamete is determined by chance.

4. When two alternate forms of a gene are present, one (the dominant allele) will mask the expression of the other (the recessive allele).

5. True-breeding (all the offspring produced are identical to the parent plant) organisms have two of the same alleles for a given trait.

Mendel´s model• Using letters to represent the different alleles, we will

assign the uppercase letter P (dominant allele) to the allele for purple and the lowercase letter p (recessive allele) to the allele for white.

• Purple-flowered plant has two alleles for purple (PP), whereas a white-flowered plant has two alleles for white (pp). All the sperm and eggs produced by a PP plant carry the P allele; all the sperm and eggs of a pp plant carry the p allele.

P

p

p

P

p

p

p

pP

P

P

P

Result: 100% P

F1

|Gametes from F1 plants F2 offspring

Sperm eggs

+P P

P

P

p

p

p p

+

+

+

PP

Pp

Pp

pp

P

P

p

p

P p

P pP P

p p

Results: 75% P, 25% p

F2

• Therefore, three types of offspring can be produced: PP, Pp, and pp. The three types occur in the approximate proportions of 1/4 PP, 1/2 Pp, and 1/4 pp.

• The actual combination of alleles carried by an organism (for example, PP or Pp) is its genotype.

• The organism´s traits, including its outward appearance, behavior, digestive enzymes, blood type, or any other observable feature, make up its phenotype.

Punnet SquarePPxPp

P

pP

P P

P

P

P

P

P p

p

Result: 50 % PP, 50 % Pp

(Genotype)

Result: 100 % P(Phenotype)

Punnet SquarePpxpp

p

p

p

pp

p p

p

p

P

P

P

Result: Genotype: 50% Pp, 50 % pp

Result:Phenotype: 50 % P, 50 % p

• Mendel´s Hypothesis can be used to predict the outcome of new types of single-trait crosses.

• Mendel hypothesized that genes on different chromosomes are inherited independently.

Problem 1• Your Grandmother (mother side) is diabetic

(Recesive) and so is your Grandfather (Father side).

• A) What are your chances of being diabetic if both of them got married with a normal homozygous?

• (Hint: You must do first your parents.)

Problem 1b• Your Grandmother (mother side) is diabetic

(Recesive) and so is your Grandfather (Father side).

• b) What are your chances of being diabetic if your Grandfather (mother side) is a normal heterozygous and your Grandmother (Father side) is a normal homozygous?

Problem 1.c

• Your Grandmother (mother side) is diabetic (Recesive) and so is your Grandfather (Father side).

• C) What are your chances of being diabetic if both of them got married with a normal heterozygous?