Mendelian Inheritance

Gregor Mendel There once was an Austrian monk named Gregor Mendel. He was born in 1822 and died in Mendel grew up on a farm and worked as a gardener in his free time. He studied physics and became a physics teacher. He also worked on breeding pea plants and bees. Mendel began to notice patterns in the traits his plants showed. In 1866 he published what he had figured out about how traits can be passed on from parents to offspring. For his discoveries, Mendel became known as the Father of Modern Genetics. Mendel’s work was particularly amazing because he figured everything out before humans knew about genes (1909) or DNA (1944).

Mendel’s Laws of Inheritance Mendel’s Laws explain patterns of inheritance. Inheritance is when something is passed down from one generation to the next (from parents to offspring). There are 3 types of alleles: Dominant – always shows up Recessive – hides behind dominant Co-dominant – both show up

Law of Segregation When gametes form during meiosis, the chromosome copies separate so that each gamete has only one copy of each chromosome. Another way to say this is that each gamete gets either one allele or the other (not both).

EXCEPTION: Nondisjuntion If meiosis doesn’t work properly and the chromosomes don’t separate, you can be missing a chromosome, or you can get an extra chromosome. Do Punnett Squares still work if meiosis doesn’t work properly? Are Punnett Squares always true, or can they sometimes give you a false result?

Law of Independent Assortment Genes sort themselves independently from each other during meiosis.

EXCEPTION: Linked Genes What Mendel didn’t know is that some genes actually are close together on the same chromosomes, so their traits are linked together.

Probability A woman is pregnant. What is the probability she will have a girl? A woman has 3 children already. All of them are boys. What is the probability that her next child will be a girl? We use probability to show the chance that a trait will be inherited (passed on).

Ms. Macway’s Family Ms. Macway has 3 sisters and no brothers. But probability says that offspring should be half (50%) boys and half (50%) girls. Why isn’t it that way in Ms. Macway’s family?