Genetics Heredity – the passing of traits from parent to offspring Heredity – the passing of traits from parent to offspring Genetics- the study of heredity Genetics- the study of heredity Part I: Mendel and the Gene Idea Part I: Mendel and the Gene Idea

I. Gregor Mendel He was a monk, a gardener, and a trained mathematician He was a monk, a gardener, and a trained mathematician 1 st to apply statistical analysis: Selective breeding was an old art… 1 st to apply statistical analysis: Selective breeding was an old art… Published his work on pea plant inheritance patterns in the 1860’s. Published his work on pea plant inheritance patterns in the 1860’s. (nothing was known about the cellular mechanisms for cellular mechanisms for inheritance…) inheritance…)

Why was the pea a good choice? 1) Cheap and available – could also feed the monks 2) Produce offspring quickly/short generation time 3) Characteristics are determined on one gene with only two alleles. Ex. Flower color: purple or white Ex. Height: tall or dwarf 4) No blending of traits 5) Can control breeding because of sex morphology

Mendel’s Questions Mendel crossed (bred) two different plants to discover what traits the offspring would show. For instance: Will a tall plant crossed with a tall plant produce all tall offspring? Mendel crossed (bred) two different plants to discover what traits the offspring would show. For instance: Will a tall plant crossed with a tall plant produce all tall offspring? If he crossed a yellow seed plant with a green seed plant, how many of the offspring will have yellow seeds and how many will have green? If he crossed a yellow seed plant with a green seed plant, how many of the offspring will have yellow seeds and how many will have green?

II. Basic genetic concepts A. Mendel studied 7 different characters in peas: Ex) Height: tall vs. dwarf Seed shape: smooth vs. wrinkled

Characters Characters – inherited features of an organism. Traits – variations of a character. Ex) character: fur color in dogs possible traits: brown, black, red

Review of chromosomes – each is made of one very long strand of DNA B. Gene – DNA sequence found on a particular chromosome, that codes for a particular trait. Chromosomes have thousands of genes and each has a particular set of genes Ex. Human chromosome 11 has the genes for making the enzymes hemoglobin, catalase, and insulin (and thousands of other genes) Ex. Human chromosome 11 has the genes for making the enzymes hemoglobin, catalase, and insulin (and thousands of other genes) In diploid organisms there are two of every gene, one from mom one from dad. In diploid organisms there are two of every gene, one from mom one from dad.

Genes express alleles Alleles are different forms of the same gene. gene that codes for brown pigment gene that codes for black pigment gene that codes for red pigment

More about alleles: Dominant allele – the “stronger” allele. In a heterozygous combination the dominant allele will be expressed. Symbol is a capital letter Ex) trait: pea plant height tall is dominant, tall = T tall is dominant, tall = T Recessive allele – “weaker” alleles. In the heterozygous individual these are hidden, and the dominant form of the trait will be expressed. Symbol is a lowercase letter Ex) trait: pea plant height dwarf is recessive, dwarf = t

Types of allele combinations HOMOZYGOUS – both genes of an individual are the same and express the same allele: HOMOZYGOUS – both genes of an individual are the same and express the same allele: Ex) Seed coat trait: Ex) Seed coat trait: wrinkled seed allele from mom and wrinkled seed allele from dad HETEROZYGOUS – each gene of the pair for a specific trait expresses two different alleles HETEROZYGOUS – each gene of the pair for a specific trait expresses two different alleles Ex) seed coat trait Ex) seed coat trait wrinkled seed allele from mom and round seed allele from dad.

Genotype: the alleles an individual has Genotype: the alleles an individual has Phenotype: the way those alleles are expressed Phenotype: the way those alleles are expressed Ex) character: pea plant height Ex) character: pea plant height Possible genotypes: T T, T t, or t t Possible genotypes: T T, T t, or t t Possible phenotypes: tall or dwarf Possible phenotypes: tall or dwarf Possible combinations for genes with two alleles: Ex) character: pea plant height Possible combinations for genes with two alleles: Ex) character: pea plant height Heterozygous = Tt, phenotype will be tall Heterozygous = Tt, phenotype will be tall Homozygous dominant = TT, phenotype will be tall Homozygous dominant = TT, phenotype will be tall Homozygous recessive = tt, phenotype will be dwarf Homozygous recessive = tt, phenotype will be dwarf

Example of an individual’s allele combinations on three gene loci

Mendel’s Experiments True-breeder – an individual that always produces offspring with the same expression for a given trait. True-breeder – an individual that always produces offspring with the same expression for a given trait. Ex) purple flower plant always produces purple flower offspring Ex) purple flower plant always produces purple flower offspring P1 – the parent generation. Two true breeders with different traits for the same character are crossed P1 – the parent generation. Two true breeders with different traits for the same character are crossed Ex) true breeding purple flower pea X true breeding white flower pea Ex) true breeding purple flower pea X true breeding white flower pea F1 – first filial generation. The offspring (progeny) of the P1 F1 – first filial generation. The offspring (progeny) of the P1 F2 – second filial generation. The offspring of two individuals from the F1 generation. F2 – second filial generation. The offspring of two individuals from the F1 generation.

Experiment Results

C. Mendel’s Conclusions Mendel’s Rules of Inheritance – generalizations made by Mendel Mendel’s Rules of Inheritance – generalizations made by Mendel Law of Segregation Law of Segregation Law of Independent Assortment Law of Independent Assortment

Mendel’s Rules of Inheritance 1) Different versions of inheritance factors (genes with different alleles) account for variations in inherited characters. 2) For each character, an organism inherits two alleles, one from each parent. 3) If two alleles differ, then one (dominant) is fully expressed, and the other (recessive) has no noticeable effect. 4) THE LAW OF SEGREGATION: The two alleles a parent has for each character segregate during gamete production.

Mendel’s Laws of Inheritance Law of Segregation - The two alleles a parent has for each character segregate during gamete production. Law of Segregation - The two alleles a parent has for each character segregate during gamete production. Ex. Heterozygote pea for flower color Ex. Heterozygote pea for flower color Pp : half of gametes get P, other half get p Pp : half of gametes get P, other half get p Refers to one Refers to one Law of Independent Assortment – for each gene the alleles separate independently of alleles for other genes Law of Independent Assortment – for each gene the alleles separate independently of alleles for other genes Ex) heterozygote for flower color (Pp) and seed color (Gg) Ex) heterozygote for flower color (Pp) and seed color (Gg) some gametes will get PG, some will get Pg, pG, or pg, in a ¼ ratio some gametes will get PG, some will get Pg, pG, or pg, in a ¼ ratio Refers to two or more genes/characters Refers to two or more genes/characters

A Test Cross WHY? To determine whether an organism that has the dominant trait is homozygous dominant or heterozygous. WHY? To determine whether an organism that has the dominant trait is homozygous dominant or heterozygous. HOW? Cross with a homozygous recessive organism. Observe the ratio of the resulting offspring. HOW? Cross with a homozygous recessive organism. Observe the ratio of the resulting offspring. How to interpret the results? How to interpret the results? 100% dominant offspring = homozygous dominant parent 100% dominant offspring = homozygous dominant parent ANY recessive offspring = heterozygous parent. ANY recessive offspring = heterozygous parent.

Patterns of Inheritance NOT revealed by Mendel’s studies Incomplete dominance Incomplete dominance Codominance Codominance Sex-linked traits Sex-linked traits Lethal alleles Lethal alleles Pleiotropy Pleiotropy Epistasis Epistasis Polygenic inheritance Polygenic inheritance

Incomplete dominance In heterozygotes both alleles are expressed resulting in a trait that blends the two. In heterozygotes both alleles are expressed resulting in a trait that blends the two. Example: flower color in snapdragons Example: flower color in snapdragons allele F R = red allele F R = red allele F W = white allele F W = white In heterozygotes (F R F W ) = pink In heterozygotes (F R F W ) = pink

Codominance In heterozygotes both alleles are expressed in separate distinguishable ways In heterozygotes both alleles are expressed in separate distinguishable ways Example: Human blood groups M,N, and MN Example: Human blood groups M,N, and MN M and N alleles expressed result in carbohydrate cell membrane markers. MM – there are only M markers NN – there are only N markers MN – there both M and N type carbs on the cell embrane

Multiple alleles There are more than two possible alleles for a character. There are more than two possible alleles for a character. Example Example: human ABO blood group 3 alleles I A, I B, and i (allele = O) O is recessive, A and B are codominant 4 possible phenotypes Type A blood (genotypes: I A I A or I A i) Type B blood (I B I B or I B i) Type O blood (ii) Type AB blood (I A I B )

Pleiotropy – one gene affects multiple phenotypes Pleiotropy – one gene affects multiple phenotypes Ex) sickle-cell causes multiple symptoms Epistasis – the outcome of one gene is affected by another gene. Epistasis – the outcome of one gene is affected by another gene. Ex) mouse color: a gene black(B)/brown(b) is expressed or not based on a second gene that allows pigment production(C)/albino (c) Polygenic inheritance -many genes affect one phenotype. Polygenic inheritance -many genes affect one phenotype. Ex) human height

Sex-linked genes Sex-linked genes – genes that are on the X chromosome show different inheritance patterns than other genes Sex-linked genes – genes that are on the X chromosome show different inheritance patterns than other genes The Y chromosome ONLY determines male gender. The X chromosome has other genes on it. The Y chromosome ONLY determines male gender. The X chromosome has other genes on it. Sex-linked traits Sex-linked traits can be passed from the mother to either sons OR daughters can be passed from the mother to either sons OR daughters can be passed from the father to ONLY daughters can be passed from the father to ONLY daughters

Mendelian Inheritance in humans: - arises from dominanat/recessive alleles on one gene loci Examples: Widow’s peak Widow’s peak Attached or free earlobes Attached or free earlobes Recessive disorders*: Ex.s) cystic fibrosis, Tay-Sachs disease, sickle-cell disease Recessive disorders*: Ex.s) cystic fibrosis, Tay-Sachs disease, sickle-cell disease Dominant disorders*: Ex.) Huntington’s Dominant disorders*: Ex.) Huntington’s *Determined with pedigree analysis, genetic testing and counseling *Determined with pedigree analysis, genetic testing and counseling

A technique for studying genetics: PCR PCR = Polymerase Chain Reaction PCR = Polymerase Chain Reaction technique for making multiple copies of DNA Procedure: 1) DNA is heated and the strands separate 2) Primers, DNA polymerase and nucleotides are added to the mixture. 3) As the DNA cools, the open strands base-pair with available nucleotides until two double helixes result. 4) Repeat steps 1-3. After 20 cycles a million DNA copies have been produced.

Uses for PCR Multiple copies of DNA are needed to Multiple copies of DNA are needed to Study genetic disease Study genetic disease Identify remains Identify remains Study prehistoric DNA Study prehistoric DNA Identify criminals Identify criminals Etc. – many applications! Every gene technology used today requires amplified DNA Etc. – many applications! Every gene technology used today requires amplified DNA

Gene Mapping – finding and identifying the location of genes. Two types Two types 1) Genetic linkage maps – show the order and 1) Genetic linkage maps – show the order and

See notes on chromosomal defects