Patterns of Inheritance

Blending theory Before 1850, scientists thought that some fluid substance in the blood of animals or in the sap of plants was the hereditary material. The combination of the parent's characteristics in the offspring was thought to occur by a "blending" of this fluid. If so, a white dog that mated with a brown dog should produce only tan puppies. A tall person who had a child with a short person should produce all "medium-size" children, etc. Clearly not the case! Even though people recognized problems with this theory, it was the top theory of the day!

http://ed.ted.com/lessons/how-mendel-s-pea-plants-helped-us-understand-genetics-hortensia-jimenez-diaz The earliest scientific experiments about how traits are inherited were done in the mid 19th century by an Austrian monk named Gregor Mendel. Mendel studied patterns of gene inheritance in everyday garden peas. He noticed that some peas were yellow and some green, some plants were short and some were tall, and that there were different colours of flowers on the plants. His lifelong studies provided the basis for the first basic understanding of heredity. Mendel discovered that traits are inherited in discrete units (we now know these to be genes). This was a long time before there was any knowledge of cells, DNA or genes

When the information is inherited from both parents is the same, i. e When the information is inherited from both parents is the same, i.e. both chromosomes carry the same allele for a particular characteristic, the chromosome is said to be homozygous. When the information inherited from both parents is different, with each chromosome in the pair carrying different instructions about a trait (different alleles) the chromosomes are referred to as heterozygous.

When two different sets of instructions are present at the same time, both characteristics can not be expressed. E.g. In the case of curly hair, the hair cannot be both straight and curly; a person has one or the other. One instruction can be “stronger” than another. This is called dominant and will be the characteristic shown in the offspring. The other gene remains hidden or masked and is called recessive.

The term PHENOTYPE is used to describe the visible characteristics in an individual that are produced by genes (i.e. straight hair). The genetic combination that produces the characteristic is the GENOTYPE. It refers to the alleles that an organism inherited from each of its parents (i.e. CC or Cc or cc).

Representing inheritance patterns with letters C = curly hair c = straight hair Genotype of the person Homozygous dominant (CC) Heterozygous (Cc) Homozygous recessive (cc) Phenotype of the person Curly hair Straight hair

To solve this type of genetics problem, called a monohybrid cross, use the following method: (Punnet Square!!) Sample Problems: In a family, a father is homozygous for curly hair. The mother is homozygous for straight hair. Determine the likelihood that they will have a curly haired child.

If one of those children, when grown up, marries and has children with another heterozygous curly haired individual, what possible phenotypes can the offspring have? Also indicate the probability of each.

Homework A woman has eye cataracts (a clouding that develops in the lens of the eye obstructing the passage of light). This is a dominant trait (E) and she is homozygous. Her husband is homozygous and normal (e). What will be the predicted genotypes and phenotypes of the children? a) Being able to roll your tongue is dominant (R), while not being able to roll your tongue is recessive (r). What will be the predicted genotypes and phenotypes of the offspring of 2 individuals who can’t roll their tongue? b) Is there any chance that these 2 individuals could produce an offspring that is able to roll his/her tongue? c) One of the offspring (called F1 generation) ends up having 2 children who can roll their tongues and 2 children who cannot. What are the genotypes of the parents of these 4 children? Draw a punnett square for this cross.

In guinea pigs, the allele for a black coat is dominant over the allele for a white coat. A black guinea pig was crossed with a white guinea pig. All if the first generation offspring have black coats. a.) Describe how you could determine whether or not the black parent is homozygous or heterozygous for the gene. Indicate which letter you would use to represent the alleles. b.) If 10 offspring were produced, indicate how many you would expect to have black coat colour if the black parent was heterozygous.