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Mendelian Genetics. The science of modern genetics had its humble beginning in the garden of an Austrian monk in the 1860s. The monk was Gregor Mendel, and his contribution to the study of heredity is discussed in every biology textbook.

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mendelian genetics
Mendelian Genetics
  • The science of modern genetics had its humble beginning in the garden of an Austrian monk in the 1860s.
  • The monk was Gregor Mendel, and his contribution to the study of heredity is discussed in every biology textbook.
  • Ironically, his work was ignored and not rediscovered until the beginning of the 20th century.
what did mendel do
He studied the characteristics of pea plants, and how they were passed from generation to generation.

The seven characteristics that he observed can be seen in the illustration to the right.

Fortunately these traits were carried on different pairs of chromosomes.

By controlling the transfer of pollen, he was able to conduct cross-fertilization experiments.

Did you know that although Mendel did his work in the 1860’s, it was ignored and not rediscovered until 1901?

What did Mendel do??
monohybrid cross
Monohybrid Cross
  • A monohybrid cross looks at a single characteristic. In this case Mendel examined flower color
analysis lead to the following four hypothesis these ideas are very important
Analysis lead to the following four hypothesisThese ideas are very important!
  • There are alternative forms of genes that account for variations in inherited of characteristics. We call these alternative forms alleles.
  • For each characteristic, an organism inherits two alleles, one from each parent. If the alleles are the same they are said to be homozygous. If the two alleles are different they are said to be heterozygous.
  • If the two alleles of an inherited gene pair differ, then one determines the organism’s appearance and is called the dominant allele. The other has no effect and is called the recessive allele.
  • A sperm or egg carries only one allele for each inherited trait because allele pairs separate from each other during meiosis and the production of gametes. This is the Law of Segregation.
the law of independent assortment
The Law of Independent Assortment
  • A dihybrid cross looks at 2 characteristics located on different pairs of chromosomes.
  • Each pair of alleles segregates independently of the other pair of alleles during meiosis. (during gamete formation)
the law of independent assortment cont
The Law of Independent Assortment (cont.)
  • When you make a dihybrid cross between two parent who are heterozygous for both traits (AaBb X AaBb) The resulting cross will give a 9:3:3:1 ratio of phenotypes.
  • 9/16 of the individuals will exhibit both dominant traits.
  • 3/16 of the individuals will exhibit one dominant and one recessive trait.
  • 3/16 of the individuals will exhibit the other combination of dominant and recessive traits
  • 1/16 of the individuals will exhibit both recessive traits.
  • A Punnett square is the easiest way to determine genotypes and phenotypes when doing dihybrid crosses
Don’t know the genotypes? Use a Testcross!Cross the unknown genotype with an individual that is homozygous recessive.
what are the chances
What are the chances?
  • Mendel’s experiments follow the laws of probability.
  • Review the rule of multiplication and the rule of addition!
  • You can use these rules to calculate the probability of any genotype resulting from a cross of known parents.
you may have a pedigreed dog but do you have a pedigree
You may have a pedigreed dog, but do you have a pedigree?
  • Yes, all of us do.
  • A pedigree is a family tree.
  • Geneticists use pedigrees to track inherited traits and conditions.
  • The tree at the right traces Martha’s Vineyard deafness in a family.
  • Pedigrees such as this can be helpful in determining both how a condition is inherited and the probability of an individuals having a trait.
inherited genetic disorders
Inherited genetic disorders
  • Genetic disorders may be recessive or dominant.
  • Examples of dominant inherited genetic disorders include Huntington’s disease and Achondroplastic dwarfism.
  • Albanism, Cystic fibrosis, and Sickle-cell anemia are recessive genetic disorders.
  • Inbreeding (mating between closely related individuals) can lead to an increase in individuals that are homozygous for recessive genes.
  • Many breeds of domestic dogs have a high incidence of disorders, such as hip displasia, due to inbreeding.
  • The breeding of endangered animals in zoos is carefully controlled to try to prevent problems related to inbreeding. That is why animals are exchanged or on temporary loan for breeding purposes


how can i know if my baby will have a genetic disorder
How can I know if my baby will have a genetic disorder?
  • This question is one that many parent have, especially if there is a family history of genetically related disease or if the mother is over 35.
  • Techniques available for genetic testing include:
    • Amniocentesis
    • Chorionic villus sampling
    • Ultrasound
    • Newborn screening
  • Many of these tests open a host of ethical considerations. What can/should one do with the information available? These issues are still being debated by society.
it may not be all or nothing incomplete dominance
It may not be all or nothing!Incomplete dominance
  • When the offspring of a cross show phenotypes that are intermediate between those of the parents, it is called incomplete dominance.
  • Unlike Mendel’s pea plants, snapdragons can have F1 flowers that are neither red or white, but pink.
  • In this case the Rr genotype produces less pigment than does the RR genotype, hence the pink color.
multiple alleles
Multiple alleles
  • You can only have two alleles for a gene (Remember there are only two chromosomes that carry that gene).
  • In a population there can be a number of alleles for a gene (though no one individual has more than two of them.
  • In the ABO blood grouping system there are 3 alleles: A,B, and O
  • That means persons will have one of the following genotypes:


A and B are carbohydrates found on the surface of the red blood cell, while O is an absence of either A or B. When both A and B are present, they are said to be codominant.

environment can effect the expression of the genes
Environment can effect the expression of the genes.
  • Crotons (Codiaeum variegatum) are common landscape plants that can be greatly effected by their environment.
  • Leaf size can be influenced by the amount of water and fertilizer.
  • Color of the leaf is determined by light conditions.
  • Clones of the exact same plant will look totally different depending upon these environmental conditions.
  • On the next slide you can see leaves from plants grown under different light conditions.
when genes are on the same chromosome mendel s laws don t work
When genes are on the same chromosome Mendel’s laws don’t work!
  • When genes are on the same chromosome they tend to be inherited together.
  • The closer two genes are located on a chromosome, the greater the chance of them being inherited together.
  • Genes located close together on a chromosome are called linked genes.
  • During meiosis,when pairs of chromosomes line up together forming tetrads, crossing over can occur between adjacent strands.
linkage and crossing over
Crossing over can lead to the formation of new combinations of genes.

Thomas Hunt Morgan and his students were leaders in the study of crossing over and its role in genetic diversity.

Determining the frequency of recombinant genotypes allowed researcher to determine the relative position of genes on a chromosome.

Linkage and Crossing over
sex determination
Sex Determination
  • Probably all of you know that the X and Y chromosomes determine sex in humans (and other mammals), however there are other

systems for sex determination that can be found in the animal kingdom.

Other systems for sex determination include the X-O system of grasshoppers and some other insects, the Z-W system of birds and a few other animals.

Sounds fairly simple…but…

Often it is a bit more complicated than this. Take a few minutes and go to the following website. Take the role of selection committee member and determine the eligibility of Jane Doe to compete. You may be surprised! and go to gender testing of female athletes

sex linked genes
Sex-linked Genes
  • In fruit flies and humans traits carried on the X chromosome are said to be sex-linked.
  • A recessive gene on the X chromosome will always be expressed in the male, since there is a single X present.
  • A female with the recessive gene on one of her two X chromosomes will be able to pass the trait to her male offspring, but will not express the trait herself. She is a carrier.
  • White eyes in Drosophila is one such sex-linked trait.
sex linked disorders
Sex-Linked Disorders
  • Sex-linked disorders primarily affect males since the defective gene is carried on the X chromosomes, and males have a single X chromosome.
  • Females would have to be homozygous for a sex-linked condition for it to be expressed, however many females are heterozygous carriers for these recessive traits.
  • Three well-known sex-linked disorders are:
    • Hemophilia
    • Duchenne muscular dystrophy
    • Red-Green color blindness