10T2K© Mendelian Genetics

Thing 1: Gregor Mendel Gregor Mendel is called the “father of genetics” because he is the first person to discover mathematical rules for inheritance. Mendel studied inheritance in pea plants. His came up with two laws of genetics that explain inheritance patterns.

Thing 2: Genes and alleles Genes are segments of DNA that contain information about making proteins. Different copies of a gene may have differences in the DNA base pairs. These alternate forms of a gene are called alleles. Homozygous organisms have two identical alleles of a gene (BB or bb). Heterozygous organisms have two different alleles of a gene (Bb).

Thing 3: Genotype and phenotype The kind of alleles you have is your genotype. For the B gene, your genotype could be, BB, Bb, or bb. Genotype = type of genes. Your physical appearance is your phenotype. Your thumb phenotype is bent or straight. Phenotype = physical appearance.

Thing 4: Dominant and recessive Sometimes, a Bb organism has the same phenotype as a BB organism. That means the B allele is dominant. That means the b allele is recessive. In this case, the only time you see the recessive phenotype is when the organism is homozygous recessive (bb).

Thing 5: Incomplete dominance Sometimes, a heterozygous organism has a phenotype half-way between the two homozygous ones. That’s called incomplete dominance. RR = red RW = pink WW = white* *These are the allele names used on the SOL.

Thing 6: Sex-linked inheritance Women have two copies of the genes on the X chromosome because they have two X chromosomes. Men have only one copy of the genes on the X chromosome because they have one X chromosome. Because of this, many sex-linked disorders show up much more often in men than in women.

Thing 7: Making gametes I—the law of segregation Gametes are sex cells = sperm and egg cells. Gametes are made by meiosis. The law of segregation says: For any pair of alleles in an organism, half its gametes will get one allele, and half will get the other allele. For a Bb organism ½ the gametes get B ½ the gametes get b

Thing 8: Making gametes II—the law of independent assortment The law of independent assortment says: Alleles of different genes end up together in a gamete by random chance. For a Bb Dd organism ¼ of the gametes get B D ¼ of the gametes get B d ¼ of the gametes get b D ¼ of the gametes get b d

Thing 9: Punnett squares Punnett squares are used to predict what kinds of offspring are possible for a set of parents. Gametes from one parent are listed at the top of the square. Gametes from the other parent are listed on the left side of the square. The middle of the square shows the possible offspring.

A sample Punnett square Bb × bb B b Bb offspring bb offspring Bb parent’s gametes bb parent’s gametes

Thing 10: Pedigrees Pedigrees are family trees that follow inheritance of certain conditions. Squares are men. Circles are women. Filled symbols have the condition. Unfilled symbols don’t have the condition.

Sample pedigrees When two parents look alike and at least Bb Bb Bb Bb B _ bb B _ bb When two parents look alike and at least one kid looks different, the kid is homozygous recessive, and the parents are both heterozygous.