Mendel and Heredity Coach Fults Chapter 8

Mendel’s Studies of Traits Heredity- passing of traits to offspring B4 DNA and chromosomes were discovered, heredity was one of the greatest mysteries of science

Mendel’s Breeding Experiments Began more than a century ago by an Austrian monk named Gregor Johann Mendel He carried out diff. experiments where he bred diff. Varieties of garden pea plants Mendel was the 1 st to develop rules that accurately predicted patterns of heredity These patterns later led to the study of Genetics

Mendel’s Breeding Experiments His parents were peasants so he was around agriculture He became an ordained priest and after 3 years he went to the University of Vienna and study math/science There he learned to study science thru experimentation and use math to explain natural phenomenas

Mendel’s Breeding Experiments Mendel later repeated the experiment of British farmer T. A. Knight that had crossed a variety of the garden pea that had purple flowers with white flowers, that resulted in all offspring with purple flowers He took 2 offspring flowers and crossed them, and it resulted in purple and white flowers; (white appeared in the 2 nd generation)

Useful Features of Peas Several traits appear in 2 forms Male and female parts are enclosed within the same flower Garden pea is small, grows easily, matures quickly, and produces many offspring

Traits Expressed in Simple Ratios Monohybrid cross- is a cross that involves 1 pair of contrasting traits He carried his experiment out in 3 steps

3 Steps for Monohybrid Cross Step 1: Mendel allowed each garden pea to self-pollinate (true-breeding) for a particular trait These true-breeding plants served as the parental generation in Mendel’s experiments. The parental generation or P-generation, are the first two individuals that are crossed in a breeding experiment

3 Steps for Monohybrid Cross Step 2: Mendel then crossed-pollinated two P generation that had contrasting forms of the trait, such as purple flowers and white flowers. Mendel called the offspring of the P generation the first filial generation or F 1 generation He then examined each F 1 plant and recorded the number of F 1 plants expressing each trait

3 Steps for Monohybrid Cross Step 3: Mendel allowed the F 1 generation to self-pollination. He called the offspring of the F 1 generation plants the second filial generation or the F 2 generation. F 2 plants were counted and analyzed

Mendel’s Results Each of Mendel’s F 1 plants showed only 1 form of the trait The contrasting form of the trait had disappeared, but when the F 1 generation was allowed to self- pollinate, the missing trait reappeared in some of the F 2 generation plants He found the ratio 3:1 to be true in all the F 2 generation plants

The Theory of Heredity People once thought that a tall plant crossed with a short plant will result in medium plant offspring This wasn’t the case Mendel proved that the dominant traits that was expressed was seen

Mendel’s Hypothesis The 4 hypotheses that he developed were based directly on the results of his experiments 1. For each inherited trait, an individual has 2 copies of the gene- one from each parent 2. There are alternate versions of genes called alleles

Mendel’s Hypothesis 3. When 2 different alleles occur together, one of them may be completely expressed, while the other may have no observable effect on the organism’s appearance 4. When gametes are formed, the alleles for each gene in an individual separate independently of one another. Thus, gametes carry only one allele for each inherited trait. When gametes unite during fertilization, each gamete contributes to one allele

Mendel’s Findings in Modern Terms Geneticists have developed specific terms and ways of representing an individual’s genetic makeup. For example letters are often used to represent alleles Dominant alleles= capital letter Recessive alleles= lower case If both the alleles are the same they are called homozygous If they are different they are called heterozygous

Examples of Homozygous and Heterozygous TT Tt Yy yy Ws WW Pp PP pp

Mendel’s Findings in Modern Terms In heterozygous individuals only the dominant allele is expressed; the recessive allele is present but not expressed Genotype- set of alleles that an individual has Phenotype- is the physical appearance of a trait Example: PP is the genotype and phenotype is purple flowers Remember that the dominant allele is always written first

Laws of Heredity 1. Law of Segregation 2. Law of Independent Assortment

Law of Segregation The first law of heredity describes the behavior of the chromosomes during meiosis At this time, homologous chromosomes and then chromatids are separated States that the 2 alleles for a trait segregate (separate) when gametes are formed

Law of Independent Assortment Mendel went on to study if the inheritance one trait influenced the inheritance of another trait He then created a dihybrid cross- shows the genotypes for 2 traits at one time. Like plant height and flower color Mendel found that for the traits he studied, the inheritance of one trait didn’t influence the inheritance of any other trait States that the alleles of different genes separate independently of one another during gamete formation

Law of Independent Assortment We now know that this law applies only to genes that are located on different chromosomes or that are far apart on the same chromosome We now know the units of heredity are portions of DNA called genes, which are found on the chromosomes that an individual inherits from its parents

Punnett Squares Animal breeders try to breed animals with very specific traits. They must be able to predict what traits will appear when 2 animals mate. One simple way of predicting the expected results (not necessarily the actual results) of the genotype or phenotype in a cross is a Punnett Square Punnett Square- is a diagram that predicts the outcome of a genetic cross by considering all the possible combinations of gametes in the cross

Punnett Squares Named for the inventor, Reginald Punnett, the simplest consists of 4 boxes

One pair of Contrasting Traits Punnett squares can be used to predict the outcome of a monohybrid cross Ends with a ratio of 3: 1

Crosses That Involve 2 traits Suppose a horticulturist has 2 traits that she wants to consider when crossing 2 plants. A cross that involves 2 traits is called a dihybrid cross. For example, she may want to predict the results of a cross between 2 pea plants that are heterozygous for seed shape( R=round, r=wrinkled) and seed color (Y=yellow, y=green) Determine the possible Genotypes!

Crosses That Involve 2 traits First consider how the four alleles from each parent (RrYy) can combine to form gametes that are either (RY,Ry,rY,ry) RrYy

Dihybrid Cross RY Ry rY ry RY Ry rY ry RRYY RRYy RrYYRrYy RRyYRRyyRryYRryy rRYYrRYyrrYYrrYy rRyY rRyyrryYrryy

Dihybrid Cross Calculate the genotype ratios Calculate the phenotype ratios

Determining Unknown Genotypes Breeders need to know if animals with dominant traits are homozygous or heterozygous for a trait Test cross- an individual whose phenotype is dominant, but whose genotype is not known, is crossed with a homozygous recessive individual For example: if a yellow seed plant (Y?) is crossed with a green seed plant (yy). If all the offspring produce yellow seeds, the offspring must be (Yy). Thus the genotype of the unknown plant must be (Yy). In reality, if 1 plant has green seeds the unknown is heterozygous.

Outcomes of Crosses Like Punnett Squares, probability calculations can be used to predict the results. Probability- is the likelihood that a specific event will occur Probability= # of one kind of possible outcome total # of all possible outcomes

Probability of a Specific Allele in a Gamete The same formula can be used to predict an allele in a gamete For seed shape could either be smooth or wrinkled. Divide that by the 2 possible outcomes. Which will give a ½ chance for either smooth or wrinkled

Probability of the Outcome of a Cross B/c 2 parents are involved in a genetic cross, both parents must be considered when calculating the probability the outcome. The allele carried by the gamete from the first parent doesn’t depend on the allele carried by the gamete from the second parent. The outcomes are independent of each other To find the probability of 2 independent outcomes multiply the separate possibilities So the probability of a penny and a nickel to both land on heads will be ½ X ½ = ¼

Probability of the Outcome of a Cross Since the combination of heads and tails can occur in two possible ways, those probabilities are added together: ¼ + ¼ = 2 / 4 or ½ Consider the possible results that can occur in 2 pea plants that are heterozygous for seed shape (Rr). R=round and r=wrinkled. The probability for each parent carrying gametes with R or r alleles is ½. The probability of the offspring with RR alleles is ½ X ½ = ¼ Similarly the probability of offspring with rr alleles is ½ X ½ = ¼

Probability of the Outcome of a Cross The combination of Rr alleles can occur in two possible ways. One parent can contribute the R allele, and the second parent the r allele, or vice versa. Thus, the probability of offspring with Rr alleles is ¼ + ¼ = ½

Inheritance of Traits What if you wanted to find out the chances of passing the trait to your children. Geneticists often prepare a pedigree, a family history that shows how a trait is inherited over several generations. They are very important in seeing if genetic disorders are likely to be passed down Carriers are people who are heterozygous for disorders but do not express it, but they can pass the disorder down

Autosomal or Sex-Linked If a trait is autosomal, it will appear in both sexes equally. Recall that an autosome is a chromosome other than X or Y sex chromosomes. If a trait is sex-linked, it is usually seen in males. A sex-linked trait is a trait whose allele is located on the X chromosome. Most sex-linked traits are recessive. B/c males only have the X chromosome, a male who carries a recessive allele on the X or Y chromosome will exhibit the sex-linked condition.

Autosomal or Sex-Linked A female who carries a recessive allele on one X chromosome will not exhibit the condition if there is a dominant allele on her other X chromosome She will express the recessive condition only if she inherits 2 recessive alleles Thus, her chances of inheriting and exhibiting a sex- linked condition are significantly less

Dominant or Recessive If a trait is autosomal dominant, every individual with the trait will have a parent with the trait If the trait is recessive, an individual with the trait can have one, two, or neither parent exhibit the trait

Heterozygous or Homozygous If individuals with autosomal traits are homologous dominant or heterozygous, their phenotype will show the dominant trait If individuals are homozygous recessive, their phenotype will show the recessive traits. 2 ppl who are heterozygous carriers of recessive mutations will not show the mutation, but they can produce children who are homozygous for the recessive allele

Complex Control of Traits Imagine a horse with red hair mates with a horse with white hair. The offspring has both red and white hair. It gets more complex than just dominant and recessive!!!!!!!!!!!!!!!!!

Traits Influenced by Several Genes When several genes influence a trait, it is said to be a polygenic trait The genes for a polygenic trait may be on the same chromosome or on a different chromosome Due to independent assortment and crossing-over in meiosis, the results are difficult to tell. Ex: skin color

Intermediate Traits Recall in Mendel’s crosses one allele was dominate and one was recessive In some organisms, an individual displays a trait that is intermediate between the 2 parents, a condition known as incomplete dominance In Caucasians, the child of a straight-haired parent and a curly-haired parents will have wavy hair. Straight and curly hair are homozygous dominant traits. Wavy hair is heterozygous and is intermediate between straight and curly

Traits Controlled by Genes with 3 or More Alleles Genes with 3 or more alleles are said to have multiple alleles. The human blood groups ABO are determined by 3 alleles: I A, I B, and I The letters A and B refer to carbohydrates found on red blood cells, if no carbs are present then its I I A, I B are both dominant over I I A, I B are codominant

Codominance 2 traits expressed at the same time

Traits Influenced by Environment An individual’s phenotype will depend on the environment it lives in Temperature will affect the artic fox color. The artic fox will produce enzymes that make pigments, that will change the white fur into a reddish-brown for the summer.

Genetic Disorders Sometimes genes are damaged and copied incorrectly, resulting in faulty proteins Mutations are rare b/c cells have efficient systems for correcting errors Harmful effects inherited by mutations are called genetic disorders Many disorders are carried in recessive alleles of heterozygous individuals 2 heterozygous ppl, can produce a homozygous individual with a mutation

Sickle Cell Anemia Recessive genetic disorder, produces defective hemoglobin (carries oxygen through the body via blood) Causes red blood cells to be sickled shape- can rupture easily carrying less oxygen Also tend to stick in vessels cutting off blood supply to organs Can protect an individual from malaria

Cystic Fibrosis Fatal recessive trait, common in Caucasians 1 in 25 have at least a copy of the gene that’s pumps chloride in and out of the cell 1 in 2,500 infants in the U.S. is homozygous for the gene The airways will get clogged with a thick mucus The ducts of the liver and pancreas become blocked No cure yet!

Hemophilia Recessive genetic disorder, impairs the bloods ability to clot Is a sex-linked trait; mutation on the X chromosome called Hemophilia A Received from his mother

Huntington’s Disease Genetic disorder caused by a dominant allele located on an autosome Mild forgetfulness and irritability- appear in individuals in their 30’s and 40’s In time, causes loss of muscle control, spasms, mental illness Usually unknowingly passed on to children

Gene Therapy Replace bad genes with healthy genes Cystic Fibrosis patients have been given DNA from a cold virus and has had success in cause those cells to stop pumping chloride But most ppl are immune to many cold viruses