Mendelian Genetics Flashcards
Mendel’s experiment
- In 1857, GregorMendel worked out the rules of inheritance through a series of brilliant experiments on garden peas.
- Early in the 20th century, Sutton and Boveri formulated the chromosome theory of inheritance:-which proposes that meiosis causes the patterns of inheritance that Mendel observed
What questions was Mendel trying to answer?
- Mendel was addressing the basic question of;
- Why offspring resemble their parents?
- How transmission of traits occurs? - During his time two hypotheses were formulated to answer these questions;
- Blending inheritance
- Inheritance of acquired characters
Mendel’s experimen
- Mendel used a Scientific Approach:
─Experimental and Quantitative - Identified to laws of inheritance
-Law of Segregation
-Principle of Independent Assortment - Mendel documented a particulate Inheritance through his experiments with garden peas.
- The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes).
Mendel’s model plant
Mendel chose a common garden pea (Pisumsativum) as his model organism because:
- It is easy to grow
- Is inexpensive
- Has a short reproductive cycle
- he could control matings
- The traits he studied were easily recognizable
trait
any physical characteristic of an individual.
Heredity
the transmission of traits from parents to their offspring
Inheritance of a single trait
His first experiments involved crossing pure lines that differed in just one trait.
-The adults in the cross were the parental generation, their offspring the F1 generation (for “first filial”).
Results: Certain Traits “Recede”
- In a cross between plants with purple flowers (PP) and white flowers (pp), all of the F1 progeny had purple flowers (Pp).
- The white flower phenotype receded.
- The results of a reciprocal cross were identical.
- reciprocal cross = Inverse cross
The Principle of Segregation
- Explains the 3:1 ratio:
- Alleles segregate into different gametes during egg and sperm formation, then come back together when an egg is fertilized by a sperm to form a zygote. - Two copies of the same allele (AA or aa) are homozygous –pure lines.
- Different copies (Aa) are heterozygous.
- A mating between parents that are heterozygous for a single trait is a monohybrid cross.
Testing the model
Mendel used self-crosseswith the F2 progenyto test his model.
-The results (F3) were as predicted.-Plants with wrinkled seeds produced only offspring with wrinkled seeds.-Plants with round seeds produced offspring with round or wrinkled seedsin the expected ratio
Mendel used dihybrid crosses to
determine whether the principle of segregation holds true if parents differ in more than one trait.
Dihybrid cross
mating between parents that are both heterozygous for two traits
The principle of independent assortment,
which states that alleles of different genes are transmitted independently of each other
- F2 phenotype ratio from dihybrid crosses supports the principle of independent assortment.
- This law appliesonly to genes on different non-homologous chromosomes or those far apart on the same chromosome.
- Genes located near each other on the same chromosome tend to be inherited together (Linked genes).
Probability laws govern Mendel’s model
- Mendel’s laws of segregation and independent assortment reflect the rules of probability.
- When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss.
- In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles.
Mendel’s Contributions to the study of heredity
- Described the basic rules of transmission genetics:-the patterns that occur as alleles pass from one generation to the next.
- Chromosome theory of inheritance arose out of the careful observations of meiosis. -Mendel’s rules can be explained by independent alignment and separation of homologous chromosomes at meiosis I.
incomplete dominance
the heterozygotes have an intermediate phenotype
*Ratio: 1:2:1
codominance
Neither allele is dominant or recessive
Pleiotropy
one gene, many phenotypic effects-E.g. Hereditary diseases have multiple symptoms (sickle cell)
epistasis
a gene at one locus alters the phenotypic expression of a gene at a second locus-E.g. in many mammals coat color depends on two genes
discrete traits (qualitatively difference).
E.g. seed color is either yellow or green -no intermediate phenotypes exist.
quantitative traits
Traits that are not discrete but instead fall into a continuum
polygenic
These traits are produced by the actions of many genes
Principle of independent assortment
- F2 phenotype ratio from dihybrid crosses supports the principle of independent assortment.
- This law appliesonly to genes on different non-homologous chromosomes or those far apart on the same chromosome.
- Genes located near each other on the same chromosome tend to be inherited together (Linked genes)
Probability laws govern Mendel’s model
- Mendel’s laws of segregation and independent assortment reflect the rules of probability.
- When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss.
- In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles
- The Multiplication andAddition Rules also apply. 5. The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities
Autosomal recessive traits
- If a phenotype results from an autosomal recessive allele, the individuals with the trait must be homozygousand the parents are heterozygous carriers.
E.g.Sickle-Cell Disease and Cystic fibrosis
Autosomal dominant traits
- If a phenotype results from an autosomal dominant allele, the individuals with the trait are either homozygousor heterozygous dominant
E.g.Achondroplasia
Linked Genes
- Linked genes deviate from the principle of independent assortment.
- This law applies only to genes on different non-homologous chromosomes or those far apart on the same chromosome.
- Genes located near each other on the same chromosome tend to be inherited together (Linked genes) because of genetic recombination
Discovery of the chromosomes
- Fruit flies (Drosophila melanogaster) became a model organism for genetic research early in the 20th century
- Thomas Hunt Morgan mated female flies with red eyes with a male with whites eye color.
- F1 generation had red eyes –he concluded that the white eye phenotype color was recessive.
- In the F2 generation, only males had white eye colo
Sex-linked inheritance
- Thomas Hunt Morgan proposed that:
- The gene for white eye color in fruit flies resides on the X-chromosome.
- The Y-chromosome does not carry an allele for this gene. -This is the hypothesis of sex-linked inheritance (or X-linkage).
- If a particular gene for a disorder resides in the Y-chromosome, then it is Y-linkage.
Sex-linked human disorders
- Many male will have the phenotypes than females.
- Sons get the allele from their mothers.
- The father cannot pass the allele to his son, bit his daughters will be carriers.
- Examples of X-linked diseases:
- Duchene Muscular disorder –progressive weakening of muscles and loss of coordination.
- Hemophilia-lack of one or more proteins required to form blood clots.
X-inactivation in females mammals
- One of the X chromosomes in mammals is inactivated.
- The inactive chromosome condenses to form a structure called a bar body
- Most of the genes in the bar body are not expressed.
