Chapter 8 Section 1 – The Origins of Genetics
Heredity- passing of characters from parents to offspring
Humans attempt to alter plants and animals to give them traits that are more useful to us.
Gregor Mendel – “father of genetics”
· Studied garden peas with purple and white flowers – cross-pollination
· When crossing these plants, all offspring were purple. When purple x purple offspring showed both purple and white flowers. The recessive trait appeared in the second generation.
Mendel’s Experiment
1. True-breeding purple should only yield purple when self-pollinated. These were the parent generation or P1. (first two individuals crossed)
2. Cross pollination of contrasting traits – purple x white – offspring are called the F1 or first filial generation. No contrast was shown in Mendel’s experiment. F1 were all purple.
3. Self-pollination of the F1 generation results in our F2 generation. The recessive allele reappeared in Mendel’s F2 generation.
Ratios are a comparison of 2 numbers and may be written as a fraction 705/224 or 705:224. Always reduce to simplest form – 705:224 à 3:1.
In all 7 of the traits that Mendel studied, the F1 offspring were 100% dominant or 4:0. In the F2 generation the ratio was 3:1.
Section 2
Mendel’s 4 hypotheses – p. 166
1. Inherited characters – individual has 2 copies of the gene to produce a particular trait
2. Alternate versions of that gene are called alleles.
3. Two alleles occur together and may be dominant - completely expressed – or recessive – masked and has no effect on observable appearance.
4. Gametes only carry one allele for each character. One allele from mother + one allele from father = 2 copies in offspring.
Letters represent alleles:
· Dominant = capital and always in front (use the first letter in the trait – ex. Tall = T)
· Recessive = lowercase of the dominant letter chosen (short = t)
When in combination, two of the same alleles (TT, tt) are called homozygous. Two different alleles (Tt) are called heterozygous. (Homo = same, hetero = different)
Units of heredity are called genes which are located on chromosomes.
Genotype – the letter combination which represents the alleles present for that trait (Ex. Pp)
Phenotype – words that describe what form of the trait is going to be seen, physical appearance (Ex. purple)
Laws of Heredity
· Law of segregation – 2 alleles separate when gametes are formed
· Law of independent assortment – the inheritance of one character has no effect on the inheritance of any other character Ex. Red hair and fair skin) The alleles for height separate independently of the alleles for flower color.
Section 3
If an individual has a dominant phenotype but their genotype is unknown, we can use a Test Cross to determine their genotype. We cross that individual with a homozygous recessive individual p. 172
Probablility – the likelihood of an event, how likely a certain genotype or phenotype might appear in the next generation, can be combined with two alleles where chances are multiplied. ½ green x ½ round = ¼ chance they will be green and round.
How inherited traits are passed through several generations can be shown on a pedigree.
· Heterozygous individuals are called carriers but do not show the symptoms of the disorder (ex. Albinism)
· Disorders may be autosomal – appears equally in both sexes – or sex-linked – allele is on the X or Y chromosome (usually the X)
· Males only have one X so if they inherit a sex-linked gene they will show the condition.
· Females may be carriers or must have 2 copies of the recessive allele to show the condition.
· Autosomal dominant – everyone who has it will have a parent with the condition
· Autosomal recessive – one, both, or neither parent may exhibit the condition
Section 4 – Complex Patterns of Heredity
Polygeneic Inheritance – several genes influence a character. (Ex. Eye color, height, weight, skin color) Genes may be on the same or different chromosomes.
Incomplete dominance – heterozygous individual is an intermediate (mixture) of the two parents (ex. Snapdragons – red x white = pink, curly hair x straight hair = wavy hair)
Multiple Alleles – genes with 3 or more alleles (Ex. Blood Types) 3 alleles – IA, IB, i
· IA and IB are dominant to i, but they are codominant when they appear together.
· Individuals still only receive 2 alleles
· IA IA , IAi = A blood type
· IB IB, IBi = B blood type
· IAIB = AB blood type
· ii = O blood type
Codominance – 2 dominant alleles are expressed at the same time – not an intermediate (ex. Black and white checkered chickens)
Environmental influence – hydrangea’s in acidic soil will bloom blue but in basic to neutral soil they bloom pink. The coat color of an artic fox and Siamese cat is affected by temperature. In humans, height/weight may be influenced by nutrition. Personality changes can be based on environmental changes. Exposure to sun affects skin color.
Genetic Disorders – changes in genetic material are mutations – rare but may have harmful effects
· Sickle cells anemia – recessive disorder – produces a defective form of the protein hemoglobin – found in rbc’s to transport oxygen. Rbc’s become sickle shaped and rupture easily which cuts off O2 supply. However, this recessive allele protects individuals from malaria (parasite that infects rbc’s)
· Cystic fibrosis – most common, fatal, hereditary disorder. 1 in 25 caucasians have at least one copy of the defective gene – makes protein needed to move Cl- into and out of cells. Airways become clogged with mucous and the ducts of the liver and pancreas become blocked. There is no known cure.
· Hemophilia – recessive sex-linked disorder on the X chromosome. Blood cannot clot.
· Huntington’s Disease – dominant disorder symptoms – mild forgetfulness and irritability, loss of muscle control, severe mental illness, death onset during 30’s or 40’s- after childbirth so unknowingly passed from one generation to the next
Treating genetic disorders – most cannot be cured – progress is being made, genetic counseling, some can be treated if found early, testing at birth is inexpensive
Gene therapy
· 1st step is to isolate the defective gene
· CF gene was isolated and successfully transferred using the cold virus as a carrier – immunity may cause rejection of the virus
· New “vehicle”- AAV - has been more accepted and the outlook from trials look promising.