Mendelian Genetics

Genetics - the study of the transmission of characteristics from parents to offspring

Heredity - the passing of traits from parents to offspring

Genes

  • units of heredity, located on chromosomes found in every cell of the body - half from maternal parent and half from paternal parent

  • more than 3 million possible combinations due to the random assortment of chromosomes in meiosis

History of Genetics

  • Aristotle believed traits were passed through the blood (the use of the term “bloodline”)

  • early naturalists believed in “hybrids” where new species resulted from breeding between other species

  • in the 1800s the common belief was a blending of the traits from both parents (blue + yellow = green)

Mendel’s Experiments

  • Gregor Mendel, an Austrian monk, performed experiments with garden peas, explaining the mechanism of gene inheritance for plants

  • pea plants were ideal for genetic experiments

  • available in many varieties for flower color, flower position, seed color, seed shape, pod shape, pod color, stem length

  • could be bred “true” - would self-fertilize and establish a set of continuing traits that were constant generation to generation, pollen from stamen could be placed on carpel to fertilize

  • the breeding cycle was less complex than other organisms and ethical concerns were negligible with plants

  • in a typical breeding experiment, Mendel would cross-pollinate between two contrasting true-breeding pea varieties

  • mating of two varieties is called a hybridization

  • mating of two varieties of true-breeding individuals tracking a single character is called a monohybrid cross

  • true-breeding parents are referred to as the P generation (parental)

  • their hybrid offspring are the F1 offspring (first filial)

  • mating the F1 generation produces the F2 generation (second filial)

Character - a heritable feature

Trait - each variant for a character

True breeding - when plants self-pollinate, all their offspring are the same variety

Hybridization - the mating or crossing of two varieties

Monohybrid cross - the term for a cross that tracks the inheritance of a single character

P generation - true breeding parents

F1 generation - the first filial generation, referring to the hybrid offspring

F2 generation - the second filial generation (F1 x F1)

The Monohybrid Cross

  • Mendel repeated his experiments with all the different traits with the same results

  • F1 generation always displayed the “dominant” factor

  • F2 generation always redisplayed the recessive trait in 25% of the offspring

What Mendel Discovered

  • regardless of parentage, purple always masked white in offspring of the true-breeding heritage

  • therefore alternate versions of a gene (alleles) account for variations in inherited characters

  • therefore for each character, an organism inherits two alleles, one from each parent

Mendel’s Law of Dominance

  • if the two alleles differ, then one, the dominant allele is fully expressed in the organism’s appearance

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

Mendel’s Law of Segregation

  • the two alleles for each character segregate during gamete production

Gene

  • the basic unit of heredity

  • made up of a sequence of DNA which is responsible for coding for a certain trait/phenotype

Allele

  • alternate forms of a gene controlling a given characteristic

  • they are found at the same positions (loci) on corresponding chromosomes

Phenotype - appearance of an organism

Genotype - genetic makeup of an organism

Homozygous - having a pair of identical alleles for a character

Heterozygous - having two different alleles for a character

Cross - mating of two organisms

Monohybrid cross - mating of two organisms, following the inheritance of one trait

Dihybrid cross - mating of two organisms, following the inheritance of two traits (always 9331)

Test cross - a test is used to determine the genotype of a dominant phenotype by crossing unknown plant with a homozygous recessive plant

Mendel’s Law of Independent Assortment

  • members of different pairs of factors behave independently and sort independently during gamete formation

Incomplete Dominance

  • the lack of a dominant gene where the offspring show an appearance in between the phenotype of two parents

  • both genes interact to produce a new phenotype (intermediate dominance) (ex: red and white make pink)

Codominance

  • both alleles are expressed in the phenotype of the offspring (ex: roan cows)

Multiple Alleles

  • Mendel followed trait that only had two alleles but most genes exist in more than two allele forms

  • for multiple alleles, capital letters are used to represent the followed with superscripts used to indicate the allele

  • ABO blood groups in humans are an example of multiple alleles of a single gene

    • a person’s blood group may be A, B, AB, or O

    • the letters represent two carbohydrates, A or B, that may be found on the surface of red blood cells

    • AB has both, O has neither

    • human body produces antibodies against the carbohydrates that are not found in their blood type —> results in clotting if the wrong blood type is received and could be fatal

    • IA = IB > i

Lethal Alleles

  • these alleles have such a detrimental effect on an organism that the organism cannot survive

  1. Lethal dominant

    1. if an organism has a lethal dominant gene it will kill them immediately —> it won’t stay long in the gene pool

  2. Lethal recessive

    1. if the organism receives two copies of the allele they will die

    2. carriers are heterozygous for the lethal allele, they are normal but will pass on the allele to offspring

  3. Lethal dominant (delayed onset)

    1. it will result in the organisms living long enough to pass on the gene

Gene Interactions

  • not all traits are determined by one gene and its multiple alleles

  • polygenic traits - traits that are determined by more than one gene like skin color, hair color, eye color, height

Epistasis

  • epistatic genes - a gene at one locus interferes with the phenotype expression of a gene at a second locus

Complimentary Interactions

  • when a gene can only be expressed in the presence of other genes, both genes have to have a dominant allele present for the final end product of the phenotype to be expressed

Pleiotropic Genes

  • when one gene affects many different characteristics in an organism

  • for example, genes that cause diseases in humans usually cause multiple symptoms

Chromosomal Theory of Inheritance

  • chromosomes carry genes - the units of hereditary structure

  • paired chromosomes separate individually of each other during meiosis

  • around 1902, scientists noted the parallels between the behavior of chromosomes and the behavior Mendel described in his pea plants

  • each chromosome contains many different genes - unlike what Mendel suggests, not all genes can segregate independently

Thomas Hunt Morgan

  • first associated a specific gene with a specific chromosome

  • worked with a species of fruit fly called Drosophilia melanogaster

    • can be bred every 2 weeks

    • has only 4 pairs of chromosomes, 3 pairs of autosomes and 1 pair of sex chromosomes

    • wild type have red eyes

    • Morgan discovered a mutant male with white eyes, this variation made it possible for Morgan to trace a gene for eye color to a specific chromosome

    • found that red eyes were dominant in F1

    • white-eyed trait only showed up in males in F2, Morgan had discovered sex-linked inheritance

  • sex-linked traits are observed more often in one sex than the other because the allele responsible for that trait is only carried on the X chromosome

  • the Y chromosome is one third the size of the X chromosome and contains about 55 genes, unlike the X chromosome which has about 900 genes

  • X-linked traits are found more often in males - they have only one copy of the X chromosome so traits won’t be masked

  • only females can be carriers of X-linked traits

  • in females, X-linked dominant traits will not be completely expressed, to maintain “homeostasis” in the body’s cells, only one of the X’s is functional in each of the cells, the other X appears as a dark spot called a “Barr body”

Pedigree Analysis

  • used to study recessive, dominant and sex-linked alleles

Autosomal Traits

  • both the males and females can exhibit the trait

  • autosomal dominant - every individual with the phenotype for that trait will have a parent with the phenotype with that trait

  • autosomal recessive - the individual with the phenotype for that trait can have one, two or neither parent exhibit the phenotype for the text

Autosomal Dominant

  • traits do not skip generations ,if a child is affected, a parent must be too

  • if parents are not affected, their children will not be affected

Autosomal Recessive

  • can skip a generation

  • unaffected parents can be carriers and have an affected child

Sex-linked Recessive

  • more male affected than females

  • an affected male receives his X-chromosome from his mother, not his father

  • an affected female must have an affected father, and all her sons will be affected

Sex-linked Dominant

  • number of males and females affected are equal

  • if a male is affected, all his daughters will be affected

Inbreeding

  • mating between relatives can increase the probability of the appearance of genetic disease

  • consanguineous

Mapping Genes

  • genetic recombination - occurs when the offspring contain a new combination of traits inherited from their two parents

  • occurs in unlinked genes as a result of independent assortment chromosomes

  • the offspring that match the parental phenotypes are called the parental types

  • the offspring that have a different combination from the parents are called recombinants

  • when half the offspring are recombinants —> 50% frequency of recombination —> occurs when two genes are located on two different chromosomes

Gene Linkage and Crossing Over

  • only have 23 pairs of chromosomes and thousands of genes, some genes must coexist on the same chromosome

  • when crosses involving two of more traits don’t yield the expected phenotype results, this is due to the linkage effect of genes on the same chromosome

  • linked genes do not assort independently because they are located on the same chromosome

  • recombination can still occur in linked genes due to crossing over during prophase in meiosis I, a crossover between homologous chromosomes breaks apart linked genes on the parental chromosome to form recombinant chromosomes that have new allele combinations

  • the closer the two linked genes are on the same chromosome, the more likely that they will stay together

  • the farther apart the two linked genes are on the chromosome, the more likely they will separate due to crossing over

Mapping Chromosomes

  • the % frequency of recombinant offspring can be used to map the relative location of genes on a chromosome

  • the higher the % recombination frequency, the farther apart the genes are on the chromosome

  • 1% recombination frequency = 1 map unit away from each other

  • using many recombination frequencies, genes can be mapped on each of the chromosomes, all map distances are additive, so many genes can be mapped at once