Concept 4: Mendelian Genetics Biology Standard 4.6 and 4.7 Sections 6.3-6.5
Genetics Overview • Genes control every trait of a living thing by controlling the formation of an organism’s proteins. • With the exception of gametes, each cell is diploid and contains two genes for each trait. • One on the maternal chromosome. • One on the paternal chromosome. • These two genes may be of the same or different forms, called alleles: an alternate forms of a gene. • i.e. blue eyes and brown eyes.
Homologous Chromosomes vs. Sister Chromatids • Homologous Chromosomes are the pair of the same chromosome (one from mom, one from dad) – they have the same genes, but not necessarily the same alleles • Sister Chromatids are the replicated copy of the chromosome – they are IDENTICAL
Gregor Mendel • Known as the father of genetics, Mendel worked in Austria with pea plants. • By crossing plants with various traits and recording meticulous results, Mendel worked out three laws of inheritance that formed the foundation of genetics.
Mendel’s discoveries: • He began with pea plants that produced only yellow or only green peas. They were known as “true breeding” and are now called “homozygous” or “purebred” for their trait—pea color. • He crossed them to create a hybrid pea. But instead of seeing yellow and green peas in the hybrid, only yellow was visible. • When he bred the yellow hybrids, the results were about 3 yellow peas for every 1 green pea. (His actual numbers were 6,022 yellow and 2,001 green…that’s a lot of peas)
From this, he concluded that there were strong and weak traits. Traits are distinguishing characteristics that can be inherited. • Mendel discovered that there were variations in genes that could be inherited. • Various forms of a single gene are called alleles. • Some alleles are dominant over others.
Law of Dominance • This law states that some alleles are dominant whereas others are recessive. • Dominant alleles = strong trait, will always be expressed • Recessive allele = weak trait, will only be expressed when the dominant allele is not present
Law of Dominance • Blue eyes- Recessive Two alleles needed Usually represented as a lower case letter (b) • Brown eyes- Dominant Only one allele needed Usually represented as an upper-case letter (B)
Law of Dominance • Since all organisms receive one chromosome from each parent, each organism contains two alleles for each gene. • If the two alleles an organism contains are of different forms, then the organism is termed heterozygous.
Law of Dominance • If the two alleles an organism contains are of the same form, then the organism is termed homozygous. • Organisms can be either homozygous dominant (two dominant alleles) or homozygous recessive (two recessive alleles).
Law of Dominance • The combination of alleles an organism possesses is known as its genotype. • An individual’s genotype can be either • Homozygous dominant, • Homozygous recessive, or • Heterozygous. • The actual trait expressed is known as the phenotype. • An individual will have either brown eyes or blue eyes. • Blue eyes- bb only Brown eyes- BB or Bb AA aa Aa
Law of Segregation • This law explains how alleles are separated during meiosis. • Each gamete produced will receive one of the alleles present in that parent organism. • Each gamete has the same chance of receiving either allele for every gene or trait. • Write the 4 combinations in your notes (under the arrows)
Law of Independent Assortment • This law states that the assortment of chromosomes for one trait does not affect the assortment of chromosomes for another trait. • Simply put, just because you pass on your dad’s chromosome #1 does not mean you’ll pass his chromosome #2, #3, #4, etc… • Any possible combination of maternal and paternal chromosomes and genes are possible when gametes are formed.
Punnett Squares • Punnett squares are used to predict the probability of passing on a certain trait. • The letters on the outside represent the possible alleles that could be passed (they are split because only ONE allele would actually be passed from each parent – remember that the sperm and egg have HALF the number of alleles) • The pairs of letters you put in the boxes represent the possible combinations when fertilization occurs.
Practice! Page 119 • #1-3 Check with partner • #4 Check with partner • #5-17 (some numbers are missing Check with partner after every OTHER problem.
Complex Inheritance Patterns Based on the Chromosome Theory of Inheritance: a basic principle that states genes are located on chromosomes and that behavior of chromosomes during meiosis accounts for inheritance patterns.
XX XY XX XY His team discovered sex determination. • More specifically they discovered that there were different chromosomes in males and females. They named them ‘X’ and ‘Y’ • Females always have XX • Males always have XY • This is why there is a 50/50 chance of having a male or female child. X Y X X
XW Y XR XR XR Y XR XRXW XRY XRXR XRY XW XRXW XRY XRXW XWY Males with white eyes!!! And they discovered traits linked to the sex chromosomes. F1 Generation • Morgan’s lab discovered a white eyed male. • It was bred with a red eyed (normal) female. • All the children had red eyes. • In the second generation, 25% had white eyes, but they were ALL MALES. F2 Generation
Sex-linked Traits • Gene for the trait is carried on a sex chromosome (usually the X-chromosome) • Females have 2 copies of the X-chromosome, so they must have 2 copies of the recessive gene to have the disorder • Females can have 1 copy of the allele for the disorder and still appear normal – we call these “carriers” • They will have a 50% chance of passing on the trait to their sons • Males only have 1 copy of the X-chromosome, so if they inherit the trait from their mother, they will ALWAYS show the trait • Examples: color-blindness, hemophilia, Duchene Muscular Dystrophy • Y-linked traits (rare) can ONLY be found in males – ALWAYS PASSED ON TO SON (only 1 copy)
Color Blindness PGM MGM MGF PGF M F B ME B
Review • Why do sex-linked traits appear more frequently in males? • Why can’t a male inherit a sex-linked trait from his father? • Draw a punnett square to determine the genotype and phenotype ratios for the children of color-blind man and a female carrier.
Gene Linkage and Crossing-over • Flies have 4 pairs of genes. • As the team bred more and more flies, they found that some genes (like wing shape and leg bristles) were always inherited together. They proposed that the genes were “linked” on the same chromosome.
Gene Linkage and Crossing-over • But sometimes, even these linked genes were mixed up. This was the result of crossing over during meiosis. • It created new allele combinations—genetic diversity!
Incomplete Dominance • The heterozygous phenotype is somewhere between the homozygous phenotypes. • Neither allele is completely dominant or completely recessive.
Incomplete Dominance • Hair Type • SS = Straight • CC = Curly • CS = Wavy (in between) What is the probability that a child will have wavy hair if her mother has wavy hair and father has curly hair?
Codominance • Both traits are fully and separately expressed. • Example: blood type • A, B, and O (also called IA, IB, and i) • Blood type is also an example of multiple alleles, having more than two alleles for one gene.
Baby 1: O Baby 2: AB Baby 3: A Make a list of the possible baby genotypes. Then make a Punnet square (or two) for each set of parents. Mr. and Mrs. Al Leel AB and AB Mr. Aaron and Mrs. Dina Haye O and AB Mr. and Mrs. Gene Pool B and O Who’s Your Daddy? Genetics
Polygenic Traits • A trait produced by two or more genes. • Examples: skin color, height • Occasionally, there is one gene that can overshadow all the others--known as epistasis. • Examples: Albinism