Mendelian Genetics and Heredity Genetics is the branch

Mendelian Genetics and Heredity Genetics is the branch of biology that deals with heredity and variation of organisms. Heredity is the passing of traits from parents to offspring.

Traits, Genes, and Alleles • DNA is wound into chromosomes • DNA encodes thousands of genes • Genes have slightly different versions from person to person • Each alternate form of a gene is called an allele. • Each organism has two alleles, one on each chromosome of a homologous pairs. • Each parent donates only one allele for each gene to an offspring

Homologous Chromosomes • Pair of chromosomes that have the same genes at the same loci, but possibly different alleles. • One set from each parent. • New combinations of genes occur in sexual reproduction • Fertilization from two parents

Mitosis

Meiosis

How do we know about genes and inheritance?

Gregor Johann Mendel (1822 – 1884) • The father of modern genetics • Austrian Monk spent 10 years in a monastery experimenting with selective breeding of peas. • Why work with peas?

Mendel looked at seven traits of garden pea (Pisum sativum).

Intro to Mendel • https: //www. youtube. com/watch? v=c. Wt 1 RFn. WNzk • Ted Ed https: //www. youtube. com/watch? v=Mehz 7 t. Cxj. SE • Sci Show: https: //www. youtube. com/watch? v=GTi. OETa. Zg 4 w

Mendel Developed Several Hypotheses: - Elements are passed from parent to offspring - Different elements control different traits - Elements unchanged during life of organism passed to offspring - Elements are passed on in the gametes of organisms

• Pure Line - a population that breeds true for a particular trait • Hybrid is as a result of crossing between two different characteristics (carries two different alleles of same gene) • Monohybrid -a hybrid that is heterozygous to a specific gene • Dihybrid - a hybrid that is heterozygous for alleles of two different genes • An F 1 hybrid (or filial 1 hybrid) is the first filial generation of offspring of distinctly different parental types • Monohybrid cross: a genetic cross involving a single pair of genes (one trait); parents differ by a single trait.

More Definitions • P = Parental generation • F 1 = First filial generation; offspring from a genetic cross. • F 2 = Second filial generation of a genetic cross • Test Cross – used to determine the genotype of an individual with a dominant phenotype

One of Mendel’s early experiments: • Mendel crossed a purple flowering plant with a white flowered plant (parents differ by a single trait = monohybrid cross )

ALL offspring of the First Filial generation (F 1): had purple flowers He let them self-pollinate to form subsequent generations F 2 , F 3 …

The F 2 generation contained: 75% purple flowers and 25% white flowers. •

F 3 hybrid cross produced same ratio as F 1

The Law of Dominance: • Mendel found one allele would mask (was dominant over) the other in the F 1 generation. • This led to Mendel's Law of Dominance which states that: • One of the factors for a pair of inherited traits (alleles) will be dominant and the other recessive, unless both factors are recessive. • Therefore, a cross between a homozygous dominant and a homozygous recessive will always express the dominant phenotype, while still having a heterozygous genotype.

Mendel’s Discoveries and Conclusions: • The concept of genes and that they occur in pairs (alleles) • Concept of Dominance • One allele may be dominant over the other • Segregation of alleles • 2 copies of a gene segregate from each other so that each gamete carries only one allele • Gametes of one trait combine irrespective of the other gene pairs (of traits)

Mendel’s Laws FIRST LAW: THE PRINCIPLE OF SEGREGATION • The two members of a gene pair (alleles) segregate (separate) from each other in the formation of gametes. Half the gametes carry one allele, and the other half carry the other allele. SECOND LAW; THE PRINCIPLE OF INDEPENDENT ASSORTMENT • Genes for different traits assort independently of one another in the formation of gametes. THIRD LAW: LAW OF DOMINANCE • one of the factors for a pair of inherited traits will be dominant and the other recessive, unless both factors are recessive.

Genes and Alleles • As we have TWO sets of chromosomes, one from mom, one from dad), we have TWO sets of genes. • Mom and dad may give us each a different form of the same gene (example: gene for brown eyes , gene for blue eyes). • The different forms of each gene is called an allele. • Often one allele may be dominant over the other, recessive allele. • The combination of alleles is called genotype. • Dominant alleles are indicated by capital letter, recessive by lower case letter.

Genotype and Phenotype • Homozygous – having two of the same alleles for a particular gene Example BB or bb • Heterozygous – having two different alleles for the same gene: Example Bb • Phenotype is the visible trait that is expressed due to our genotype BB, Bb – Brown eyes bb – blue eyes

The Law of Dominance: • A one-eyed purple people eater is crossed with a two-eyed purple people eater. • All of their offspring have two eyes. • Which trait is dominant?

The Law of Dominance: • EE = two eyes • Ee = two eyes • ee = one eye What’s the genotype of the offspring if each parent was purebred? EE x ee • What if you crossed the offspring with each other? • How many new offspring would you expect to have two eyes?

Mendel didn’t know about chromosomes and genes or meiosis but figured out that there was some factor for each trait Eg. Tall plant: T vs. t • Mendel did the same experiment for all seven characteristics. • In each F 1 generation: one trait disappeared and then showed up again in the F 2 plants. • In each F 2 generation: 75% had one trait and 25% had the other trait. • Based on these observations, Mendel formulated the Law of Segregation….

The Law of Segregation • There are two factors controlling a given characteristic (alleles), and these factors separate and go to different gametes when a parent reproduces. • We now know that the segregation of genes occurs during meiosis in eukaryotes, which is a process that produces reproductive cells called gametes.

The Gene-Chromosome Theory • It wasn’t until 1900 that Mendel’s discoveries were fully understood. • New information on cells allowed scientists to realize Mendel’s factor were genes which are found on chromosomes inside the cell.

Can organisms with different genotypes have the same phenotypes?

Yes: Example using Monohybrid Cross • Crossing two pea plants that differ in stem size, one tall one short T = allele for Tall t = allele for dwarf TT = homozygous tall plant t t = homozygous dwarf plant TT tt

Monohybrid cross for stem length: P = homozygous plants: F 1 generation is heterozygous: TT tt (tall) (dwarf) Tt (all tall plants)

Amoeba Sisters: Alleles and Genes • https: //www. youtube. com/watch? v=pv 3 Kj 0 Uji. LE

Punnett Squares

Using a Punnett Square We use the Punnett square to predict the genotypes and phenotypes of the offspring Cross a TT (Tall) plant with a t t (short) plant TT tt

Secret of the Punnett Square • Key to the Punnett Square: • Determine the gametes of each parent… • How? By “splitting” the genotypes of each parent: If this is your cross T T t t The gametes are: T T t t

Once you have the gametes… T T t t Tt Tt

Punnett square 1. "split" the letters of the genotype for each parent & put them "outside" the p-square 2. determine the possible genotypes of the offspring by filling in the p-square 3. summarize results (genotypes & phenotypes of offspring) TT tt t t T T Tt Tt Genotypes: 100% T t Tt Phenotypes: 100% Tall plants Tt

Cross the F 1 generation

PUNNET SQUARE • Tt x Tt • Homozygous – Individual has both alleles the same. TT – homozygous tall plant. tt – homozygous short • Heterozygous – Has one of each allele. Tt – heterozygous tall

Monohybrid cross: F 2 generation • If you let the F 1 generation self-fertilize, the next monohybrid cross would be: Tt (tall) Genotypes: 1 TT= Tall T t 2 Tt = Tall 1 tt = dwarf T TT Tt Genotypic ratio= 1: 2: 1 Phenotype: 3 Tall t Tt tt 1 dwarf Phenotypic ratio=

• Genotype – The combination of alleles that an organism possesses. • TT, Tt and tt are different genotypes • Phenotype – The outward appearance of an organism • The phenotype for TT would be tall The phenotype for Tt would be tall. The phenotype for tt would be short.

Test cross When you have an individual with an unknown genotype, you do a test cross with a homozygous recessive individual. For example, a plant with purple flowers can either be PP or Pp… therefore, you cross the plant with a pp (white flowers, homozygous recessive) P ? pp

Draw a Punnett Square for this test cross.

Test cross • If you get all 100% purple flowers, then the unknown parent was PP… p • If you get 50% white, 50% purple flowers, then the unknown parent was Pp… p p p P P Pp Pp P p Pp pp

Dihybrid Cross (involves two traits) • Cross between two organisms heterozygous for the same traits. • Used to investigate two different genes

Dihybrid Cross RR = Round rr = Wrinkled YY = Yellow Yy = Green

Dihybrid Cross of F 2: When F 1 generation is allowed to self pollinate, mendel observed 4 phenotypes Phenotype Ratio = 9: 3: 3: 1 But 9 different genotypes

Law of Independent Assortment Genes for different traits are sorted separately from one another so that the inheritance of one trait is not dependent on the inheritance of another (dihybrid cross). Different traits are inherited independently (separately) of one another.

Law of Independent Assortment:

Dihybrid test cross? ? If you had a tall, purple plant, how would you know what genotype it is? tt pp ? ? 1. 2. 3. 4. TTPP TTPp Tt. PP Tt. Pp

Chromosome Theory of Inheritance 1. Chromosomes contain the genetic material, which is transmitted from parent to offspring and from cell to cell. Genes are found on chromosomes. 2. Chromosomes are replicated and passed from parent to offspring. They are also passed from cell to cell during the multicellular development of an organism. Each type of chromosome retains its individuality during cell division and gamete formation. 3. The nucleus of a diploid cell contains two sets of chromosomes, which are fond I homologous pairs. One member of each pair is inherited from the mother and the other from the father. The maternal and paternal sets of homologous chromosomes are functionally equivalent; each set carries a full complement of genes.

• At meiosis, one member of each chromosome pair segregates into one daughter nucleus and its homologue segregate into the other daughter nucleus. Each of the resulting haploid cells contains only one set of chromosomes. During the formation of haploid cells. , the members of different chromosome pairs segregate independently of Each other. • Gametes are haploid cells that combine to form a diploid cell during fertilisation, with each gametes transmitting one set of chromosomes to the offspring. In animals, one set comes from the mother and the other set comes from the father.

Summary of Genetics • Chromosomes carry hereditary info (genes) • Chromosomes (and genes) occur in pairs • New combinations of genes occur in sexual reproduction • Monohybrid vs. Dihybrid crosses • Mendel’s Principles: • Dominance: one allele masks another • Segregation: genes become separated in gamete formation • Independent Assortment: Members of one gene pair segregate independently from other gene pairs during gamete formation

Practice

Patterns of Inheritance The manner in which a gene is transmitted

5 Patterns of Inheritance • Incomplete Dominance • Co dominance • Autosomal Dominant • Autosomal Recessive • X-linked

Beyond Mendelian Genetics: Incomplete Dominance Mendel was lucky! Traits he chose in the pea plant showed up very clearly… One allele was dominant over another, so phenotypes were easy to recognize. But sometimes phenotypes are not very obvious…

Incomplete Dominance – Neither trait expressed Snapdragon flowers come in many colors. If you cross a red snapdragon (RR) with a white snapdragon (rr) You get PINK flowers (Rr) RR Heterozygous genotype results in an Intermediate phenotype (blended trait) Rr rr

Incomplete dominance When F 1 generation (all pink flowers) is self pollinated, the F 2 generation is 1: 2: 1 red, pink, white Incomplete Dominance R r R R Rr Rr rr

Incomplete dominance What happens if you cross a pink with a white? A pink with a red?

Incomplete Dominance • Both alleles contribute to phenotype • Mixing of parental traits • Eg a gene for hair texture: curly hair allele from one parent and straight-hair allele from other wavy hair

Codominance • Both alleles expressed, NOT mixed • Each trait is retained

Co-dominance • Both traits expressed. • EG. An animal with allele for white hair and an allele for red hair produce a roancoloured coat (both white hairs and red hairs, not blended).

Codominance

Practice

Codominance with Multiple Alleles: Blood Type • Multiple alleles control the ABO blood groups in humans. • The A and B alleles are codominant to each other, and the O allele is recessive • A Type x B Type = A and B type • A, B, O alleles A, B, AB, O blood

Blood Type

Polygenic Inheritance (multifactorial inheritance) • More than one gene involved in determining a particular characteristic, e. g. height or skin colour. • not to be confused with multiple alleles • Say that the genes A and B control human height. The dominant allele would be for tall and the recessive allele will be for short. However, the total height depends on the amount of tall genes you have: • AABB • a. ABB, Aa. BB, AAb. B, AABb • Aa. Bb, a. ABb, AAbb, Aab. B, a. ABb, aa. BB • Aabb, a. Abb, aa. Bb, aab. B • aabb