Genetic Terminology: (In Notebook After the Test)

Chapter 8- Genetics

Genetic Terminology: (in notebook after the test)

 Trait - any characteristic that can be passed from parent to offspring  Heredity - passing of traits from parent to offspring  Genetics - study of heredity  Alleles - two forms of a gene (dominant & recessive)  Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R)  Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r)  Genotype - gene combination for a trait (e.g. RR, Rr, rr)  Phenotype - the physical feature resulting from a genotype (e.g. tall, short)  Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (e.g. RR or Rr); also called pure  Heterozygous genotype - gene combination of one dominant & one recessive allele (e.g. Rr); also called hybrid  Monohybrid cross - cross involving a single trait  Dihybrid cross - cross involving two traits  Punnett Square - used to solve genetics problems

Gregor Mendel: (notes)

 Austrian monk  Studied science & math at the University of Vienna  Formulated the laws of heredity in the early 1860's  Did a statistical study of traits in garden peas over an eight year period

Why peas, Pisum sativum? (discussion)

 Can be grown in a small area  Produce lots of offspring  Produce pure plants when allowed to self-pollinate several generations  Can be artificially cross-pollinate

Mendel's Experiments: (notes)

 Mendel studied simple traits from 22 varieties of pea plants (seed color & shape, pod color & shape, etc.)  Mendel traced the inheritance of individual traits & kept careful records of numbers of offspring  He used his math principles of probability to interpret results  Mendel studied pea traits, each of which had a dominant & a recessive form (alleles)  The dominant (shows up most often) gene or allele is represented with a capital letter, & the recessive gene with a lower case of that same letter (e.g. B, b)  Mendel's traits included:

a. Seed shape --- Round (R) or Wrinkled (r) b. Seed Color ---- Yellow (Y) or Green (y) c. Pod Shape --- Smooth (S) or wrinkled (s) d. Pod Color --- Green (G) or Yellow (g) e. Seed Coat Color --- Gray (G) or White (g) f. Flower position --- Axial (A) or Terminal (a) g. Plant Height --- Tall (T) or Short (t) h. Flower color --- Purple (P) or white (p)

 Mendel produced pure strains by allowing the plants to self-pollinate for several generations

 These strains were called the Parental generation or P1 strain  Mendel cross-pollinated two strains and tracked each trait through two generations (e.g. TT x tt )

Trait - plant height Alleles - T tall, t short

P1 cross TT x tt genotype -- Tt t t phenotype -- Tall T Tt Tt genotypic ratio --all alike T Tt Tt phenotypic ratio- all alike

 The offspring of this cross were all hybrids showing only the dominant trait & were called the

First Filial or F1 generation

 Mendel then crossed two of his F1 plants and tracked their traits; known as an F1 cross

Trait - plant height Alleles - T tall, t short

F1 cross Tt x Tt genotype -- TT, Tt, tt T t phenotype -- Tall & short T TT Tt genotypic ratio --1:2:1 ct Tt tt phenotypic ratio- 3:1

 When 2 hybrids were crossed, 75% (3/4) of the offspring showed the dominant trait & 25% (1/4) showed the recessive trait; always a 3:1 ratio

 The offspring of this cross were called the F2 generation

 Mendel then crossed a pure & a hybrid from his F2 generation; known as an F2 or test cross

Trait - Plant Height Alleles - T tall, t short

F2 cross TT x Tt F2 cross tt x Tt T t T t T TT Tt t Tt tt T TT Tt t Tt tt

genotype - TT, Tt genotype - tt, Tt phenotype - Tall phenotype - Tall & short genotypic ratio - 1:1 genotypic ratio - 1:1 phenotypic ratio - all alike phenotypic ratio - 1:1  50% (1/2) of the offspring in a test cross showed the same genotype of one parent & the other 50% showed the genotype of the other parent; always a 1:1 ratio Results of Mendel's Experiments:

 Inheritable factors or genes are responsible for all heritable characteristics  Phenotype is based on Genotype  Each trait is based on two genes, one from the mother and the other from the father  True-breeding individuals are homozygous ( both alleles) are the same  Law of Dominance states that when different alleles for a characteristic are inherited (heterozygous), the trait of only one (the dominant one) will be expressed. The recessive trait's phenotype only appears in true-breeding (homozygous) individuals  Law of Segregation states that each genetic trait is produced by a pair of alleles which separate (segregate) during reproduction  Law of Independent Assortment states that each factor (gene) is distributed (assorted) randomly and independently of one another in the formation of gametes

Other Patterns of Inheritance:

 Incomplete dominance occurs in the heterozygous or hybrid genotype where the 2 alleles blend to give a different phenotype  Flower color in snapdragons shows incomplete dominance whenever a red flower is crossed with a white flower to produce pink flowers

 In some populations, multiple alleles (3 or more) may determine a trait such as in ABO Blood type  Alleles A & B are dominant, while O is recessive. This is known as codominance because both traits are displayed

Genotype Phenotype IOIO Type O IAIO Type A IAIA Type A IBIO Type B IBIB Type B IAIB Type AB  Polygenic inheritance occurs whenever many variations in the resulting phenotypes such as in hair, skin, & eye color  The expression of a gene is also influenced by environmental factors (example: seasonal change in fur color)

Pedigrees:

. Also called a family tree . Squares represent males and circles represent females . Horizontal lines connecting a male and female represent mating . Vertical lines extending downward from a couple represent their children . A shaded symbol means the individual possess the trait . Half-shaded symbols are carriers

Sex Linkage:

If a trait is autosomal, it will appear in both sexes equally.

If a trait is sex linked, it will appear more frequently in males because the trait is located in the x chromosome. Most sex linked traits are recessive, and since a male only has one x chromosome, a male who carries the trait will express it. A female who carries the recessive trait will not express the trait if there is a dominant allele on her other X chromosome.

. In humans, another well-known X-linked traits is hemophilia (free bleeders that lack clotting factors in their blood) . One of the most famous genetic cases involving hemophilia goes back to Queen Victoria who was a carrier for the disorder and married Prince Albert who was normal . Their children married other royalty, and spread the gene throughout the royal families of Europe

Linked genes:

. Each chromosomes has 1000's of genes . All genes on a chromosome form a linkage group that stays together except during crossing-over . Some genes located on the same chromosome tend to be inherited together . Linked genes were discovered by Thomas Hunt Morgan while studying fruit flies . Linked alleles do not obey Mendel's laws because they tend to go into the gametes together . Crosses involving linked genes do not give same results as unlinked genes

Environmental effects:

. Sometimes genes will not be fully expressed owing to external factors. Example: Human height may not be fully expressed if individuals experience poor nutrition.

Gene interaction:

. Many biological pathways are governed by multiple enzymes, involving multiple steps. If any one of these steps are altered. The end product of the pathway may be disrupted.

Continuous Variation

. Many traits may have a wide range of continuous values. Eg. Human height can vary considerably. There are not just "tall" or "short" humans