Genetics The study of heredity: Passing genetic traits from one generation to next.
Two basic questions about heredity: 1.What determines the traits that an organism has? Examples: hair color, behavior 2.How are traits transmitted from one generation to the next? (Why do we resemble our parents?) Answer: Genes (of course)
Gregor Mendel “Father of Genetics” Austrian Monk Studied heredity in 1850s Used garden peas
Mendel studied peas because their phenotypes were easy to identify. Phenotype - the outward, physical appearance of a particular trait
Mendel's pea plants exhibited the following phenotypes: - round or wrinkled seeds - yellow or green seeds - red or white flowers - tall or short plants
P (parent) generation were “pure-bred” for traits…they only carried the trait they showed! Mendel did not know about chromosomes, genes or DNA! He suggested that some “factors” were passed from parents to offspring. “Factors” that Mendel talked about were actually ALLELES (versions of a gene)
“Factors” (alleles) are inherited from your parents. You get 1 allele for each trait from each parent. Ex – You have 2 genes for tongue rolling. You can have 2 alleles for the ability to roll the tongue, 2 alleles for inability to roll, or 1 of each allele.
Genotype Tongue Rolling (Dominant) roll/not roll Widow's Peak (D) - just like Eddie Munster yes/no Wet ear wax (D) - stick your finger in to check wet/dry L/R interlocking finger (D) - without thinking, clasp your hands together, is the right thumb over the left, or vice versa? L/R or R/L Attached earlobes (D) - ask a neighbor or check out the mirror yes/no Hitchhiker Thumb (r) - does it bend back at a 90 angle yes/no PTC tasting (D) - I'll have some test paper in class yes/no Chin fissure (D) - like actor Michael Douglas yes/no Darwin tubercle (D) - little bump on the inside of the ear yes/no S-methylthioester detection (Recessive) - can you smell asparagus odor in urine? yes/no Pigmented iris (D) - any color but blue yes/no Freckles (D) yes/no Polydactyl (D) - more than five fingers and/or toes yes/no Dimples (D) yes/no Wooly hair (R) yes/no Long eyelashes (> 1cm; D) yes/no Short big toe (D) - the big toe is shorter than your second one yes/no
Mendel’s laws 1)Law of Dominance – some alleles show and some alleles are “hidden”. The traits that mask other traits are dominant traits. EX: Tall plant X Short plant all offspring are Tall Tall is the dominant trait!
Use capital letter to represent dominant allele Use lower case letter to represent recessive allele EX: T = tallt = short
REMEMBER… Each organism has a pair of alleles (genes) for each trait (diploid) TT = homozygous dominant (tall) Tt = heterozygous (tall, but carries an allele for short) tt = homozygous recessive (short)
2) Law of Segregation – during meiosis, one member of a gene pair separates into different gametes. Therefore, each gamete only carries one member of the gene pair.
3) Law of Independent Assortment – Gametes unite at random and irrespective of the other gene pairs involved.
Inheritance follows the rules of probability –The rule of multiplication and the rule of addition can be used to determine the probability of certain events occurring Mendel ’ s principles reflect the rules of probability F 1 GENOTYPES Bb female F 2 GENOTYPES Formation of eggs Bb male Formation of sperm 1/21/2 1/21/2 1/21/2 1/21/2 1/41/4 1/41/4 1/41/4 1/41/4 BB BB B B b b b b bb Figure 9.7
Using symbols we can depict the cross of tall and short pea plants in the following manner:
Allele - one alternative form of a given gene pair. Tall and dwarf are the alleles for the height of a pea plant. More than two alleles can exist for any specific gene, but only two of them will be found within any individual.
A human male has one X chromosome and one Y chromosome A human female has two X chromosomes Whether a sperm cell has an X or Y chromosome determines the sex of the offspring SEX CHROMOSOMES AND SEX-LINKED GENES Chromosomes determine sex in many species
All genes on the sex chromosomes are said to be sex-linked –In many organisms, the X chromosome carries many genes unrelated to sex –Fruit fly eye color is a sex-linked characteristic Sex-linked genes exhibit a unique pattern of inheritance Figure 9.22A
–Their inheritance pattern reflects the fact that males have one X chromosome and females have two
Most sex-linked human disorders are due to recessive alleles –Examples: hemophilia, red-green color blindness –A male receives a single X-linked allele from his mother, and will have the disorder A female has to receive the allele from both parents to be affected Sex-linked disorders affect mostly males Figure 9.23A
Hemophilia - sex linked trait A high incidence of hemophilia has plagued the royal families of Europe Figure 9.23B Queen Victoria Albert AliceLouis AlexandraCzar Nicholas II of Russia Alexis
In a population, multiple alleles often exist for a characteristic –The three alleles for ABO blood type in humans is an example Many genes have more than two alleles in the population
Figure 9.13 –The alleles for A and B blood types are codominant, and both are expressed in the phenotype Blood Group (Phenotype) O Genotypes Antibodies Present in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left OABAB A B ii I A or I A i I B or I B i I A I B Anti-A Anti-B Anti-A