Genetics

Segregation of chromosomes during meiosis causes predictable patterns of

inheritance.

• REVIEW: During Meiosis, segregation of maternal and paternal chromosomes is random.

• Because segregation of chromosomes is random, inheritance of single genes follows the rules of probability.

Simple Example: Single celled eukaryote with diploid number 2.

• One homologous pair of chromosomes• Gene for Protein X. • There are two variants of protein X (small

differences in the DNA coding for protein X)– One variant is functional (X+)– The other variant is non-functional (X-)

• If an individual has both alleles it will express the functional protein.

X+ X-

Crossing individuals with different genotypes:

• Parent one is homozygous for the functional allele

• This is how we denote the genotype of parent one : – X+X+

• Parent two is homozygous for the non-functional allele: X-X-

First: predict the possible gametes that each individual will produce

• Meiosis produces haploid gametes.• Parent One Parent Two

X+X-

X+ X-

X+ X-

What are the possible genotypes of the offspring produced by parent one and two?• Fertilization:

• Genotype of offspring: X+X-

• Phenotype of offspring: All will express functional protein X

What if two heterozygotes are crossed?

• What are the possible gametes that can be produced by a heterozygous individual?

X+ or X- X+ or X-

Fertilization

• Possible offspring genotypes: X+X+, X+X-, X+X-, X-X-

• What are the proportions of each phenotype?• ¾ or 75% will express the functional protein• ¼ or 25% will express the non-functional protein

Parent 1 gametes

Parent 2 gametes

X+ X- X- X+

Gregor Mendel observed these patterns when he performed test crosses with Pea Plants.

• Mendel was able to discover some important features of inheritance- not knowing about DNA, chromosomes, or meiosis

• HOW?• He used mathematical analysis to show that

inheritance patterns followed rules of probability. • He deduced features of chromosomal and genetic

inheritance using the proportions he observed

“In this generation, along with the dominating traits, the recessive ones also reappear, their individuality fully revealed, and they do so in the decisively expressed average proportion of 3:1, so that among each four plants of this generation three receive the dominating one and one the recessive characteristic.”

Lived from 1822 to 1884

Significant and enduring understanding from Mendel’s work

• Traits are inherited as discrete units “genes”• Traits are not blended; genes remain distinct units• Mendel's Law of Segregation• Mendel’s Law of Independent Assortment

Mendel’s Law of Segregation:

1. Individuals have two of each gene2. Individuals pass only version of each gene to

their offspring

What is the basis for this law in terms of chromosomes and the process of Meiosis?

• Most Eukaryotes are Diploid; they have a pair of homologous chromosomes with the same genes on them.

• Meiosis produces haploid gametes with one of each type of chromosome.

Mendel discovered the Law of Independent assortment by studying inheritance patterns of dihybrid crosses.

-Patterns of inheritance of two separate single gene traits.

Mendel’s Law of Independent Assortment:

• Genes assort independently• For genes located on different chromosomes:

the inheritance of one gene does not influence the inheritance of another gene.

• What is the basis for this law in terms of chromosomes and the process of meiosis?

Basic Rules of Probability• For two independent events (a and b), the

probability that two events will occur at the same time is:– The and rule:– Probability of a and b= P(a) x P (b)

• The probability of a or b (exclusive) occurring is:– The or rule• Probability of a or b = P(a) + P(b)

Rules of probability can be applied to analyze the passage of single genes traits from parent to

offspring

• What is the probability of a heterozygous individual (genotype Aa) producing a gamete with the A allele?

• What is the probability of a heterozygous individual (Aa) producing a gamete with A or a alleles?

More application of probability

• What is the probability of two heterozygous individuals (Aa) producing offspring with the genotype aa?

What is the probability of a homozygous recessive and heterozygote producing

offspring with the dominant phenotype

Two gene inheritance and probability

• Use the probability rules to predict the outcome of inheritance of two genes located on different chromosomes.

• Parents: YyGg x YyGg• What are the possible offspring genotypes and

phenotypes• What will be the proportion of each genotype

and phenotype?

What were the genotypes of the parents?

• A scientist crossed two parents with unknown genotypes and observed the following phenotypes in the offspring:

• 75 white flowers• 225 Purple flowers• Which trait is dominant, which trait is

recessive?• What were the genotypes of the parents?

What was the genotype and phenotype of the parents?

• Two parents of unknown genotype were crossed. The following phenotypes were observed:

• 30 Yellow and Wrinkled seed coat• 10 Green and Wrinkled seed coat• 90 Yellow and Round seed coat• 30 Green and Round seed coat

Sex linked traits are traits that are inherited via the X or Y chromosome.

• Sexual dimorphism: characteristic that differ between genders.– Example: In placental mammals and marsupials the SRY gene

determines development of testes. • SRY is located on the Y chromosome– The SRY gene codes for a transcription factor that binds

Regulatory DNA and influences what proteins the cells will make

• If SRY gene is non functional, XY Female develops (Swyer Syndrome)

• Translocation of the SRY gene to the X chromosome (due to incorrect crossing over event between X and Y) causes XX male to develop.

Sex-linked inheritance patters• Example: hemophilia is an X linked recessive

disorder• Conventions: – XH (normal functional allele) – Xh (non functional allele –causes hemophilia)

• The following parents are expecting a daughter: XHXh and XHY. What is the probability that she will have hemophilia?

• If the same couple is expecting a son, what is the probability he will have hemophilia?

Gene linkage

• Genes on different chromosomes follow the rules of segregation and independent assortment.

• Genes located on the same chromosome tend to assort together to a certain extent.

• The closer two genes are on the same chromosome, the more likely they will be inherited together.

• This is called gene linkage

Data of parent and offspring inheritance can be used to predict whether genes are linked.

• Possibilities: monohybrid cross, dihybrid cross, sex-linked trait, genes linked on same homolgous chromosome.

• Monohybrid cross• Dihybrid cross• Sex-linked traits• Linked genes• Genetic diseases• Predict the pattern of inheritance based on

parent and offspring genotype and phenotype