Patterns of Inheritance Chapter 9 Patterns of Inheritance

HERITABLE VARIATION AND PATTERNS OF INHERITANCE HEREDITY is the transmission of traits from one generation to the next. Gregor Mendel Worked in the 1860s Was the first person to analyze patterns of inheritance Described the fundamental principles of genetics

Figure 9.1 Figure 9.1 Gregor Mendel.

At the monastery garden Mendel studied garden peas because they Are easy to grow Are easily manipulated (easy to cross them with each other) Have many easily recognized trait variations Tall or short plants (trait = Puple or white flowers (trait = Round or wrinkled seeds (trait = Green or yellow seeds (trait = Etc... Please note: what your book calls “characters,” I will call “traits”

HYBRIDS are the offspring of two different true-breeding varieties. Mendel Created true-breeding varieties of plants What does that mean? Then he would cross different true-breeding varieties, collect the seeds, plant the seeds, and look at the results The big new thing he did was to COUNT the results HYBRIDS are the offspring of two different true-breeding varieties. The parental plants are the P GENERATION. Their hybrid offspring are the F1 GENERATION. A cross of the F1 plants forms the F2 GENERATION.

Mendel performed many experiments. (1000s of plants) He tracked the inheritance of traits that occur as two alternative forms (like yellow or green seeds). White Purple Recessive Dominant Green Yellow Terminal Axial Wrinkled Round Seed shape Seed color Flower position Flower color Pod color Pod shape Stem length Inflated Constricted Tall Dwarf Figure 9.4

Monohybrid Crosses A monohybrid cross is a cross between parent plants that differ in only one trait. Like flower color  Purple flowers F1 Generation White flowers P Generation (true-breading parents) All plants have purple flowers F2 Generation Fertilization among F1 plants (F1  F1) of plants have purple flowers have white flowers Figure 9.5-3

Mendel developed four hypotheses from the monohybrid cross: 1. There are alternative versions of genes, called ALLELES. 2. For each trait, an organism inherits two alleles, one from each parent. An organism is HOMOZYGOUS for that gene if both alleles are identical. An organism is HETEROZYGOUS for that gene if the alleles are different.

3. The alleles come in two general forms: The DOMINANT allele. If an organism gets either one or two copies of the dominant allele, the organism will “show” the dominant trait. For example: homozygous dominant; RR or heterozygous; Rr Dominant traits are not necessarily normal or more common Example: having 5 fingers is recessive, 6 fingers (polydactyl) is dominant The RECESSIVE allele. An organism has to get TWO copies of the recessive allele to show the recessive trait. For example: homozygous recessive; rr

DOMINANT TRAITS RECESSIVE TRAITS Freckles Widow’s peak Free earlobe No freckles Straight hairline Attached earlobe Figure 9.12 Figure 9.12 Examples of inherited traits thought to be controlled by a single gene.

4. Gametes carry only one allele for each trait. The two members of an allele pair segregate (separate) from each other during the production of gametes (meiosis). This happens during Metaphase I of meiosis Example: you have TWO eye color genes, one from mom and one from dad. But when you make a sex cell, you don’t put both of those genes in one sex cell...they get separated. You put one of the eye color genes in one sex cell, and the other eye color gene in the other sex cell This concept is the LAW OF SEGREGATION.

Geneticists distinguish between an organism’s physical traits and its genetic makeup. An organism’s physical traits are its PHENOTYPE. Blue eyes, brown hair, dark skin, tall, short, etc... An organism’s genetic makeup is its GENOTYPE. The letters we use to describe the organism’s genes. Examples: AA, Rr, Jj, ff, cc, Dd, etc...

How can we determine what children will result from a mating (a cross) of two parents? First we figure out what gametes each parent could produce, then we set up a Punnett Square. A PUNNETT SQUARE highlights the possible combinations of gametes and offspring that result from each cross. What gametes could you produce if you had the genotype Ff? F or f What gametes could you produce if you had the genotype DD? D or D

Figure 9.6-3 P Generation Genetic makeup (alleles) Purple flowers White flowers PP pp Alleles carried by parents Gametes All P All p F1 Generation (hybrids) Purple flowers Alleles segregate All Pp 1 2 1 2 Gametes P p F2 Generation (hybrids) Sperm from F1 plant P p P Eggs from F1 plant PP Pp p Pp pp Phenotypic ratio 3 purple : 1 white Genotypic ratio 1 PP : 2 Pp : 1 pp Figure 9.6-3 Figure 9.6 The law of segregation. (Step 3)

Mendel’s Law of Independent Assortment A DIHYBRID CROSS is the crossing of parental varieties differing in two traits. Like seed color AND seed shape (two different traits)

Actual results (support hypothesis) Independent assortment P Generation RRYY rryy Gametes RY ry F1 Generation RrYy Sperm F2 Generation RY rY Ry ry RY RRYY RrYY RRYy RrYy rY 16 9 Yellow round RrYY rrYY RrYy rrYy Eggs Ry 16 3 Green round RRYy RrYy RRyy Rryy 16 3 Yellow wrinkled ry RrYy rrYy Rryy rryy 1 16 Green wrinkled Actual results (support hypothesis) Figure 9.8 Testing alternative hypotheses for gene assortment in a dihybrid cross.

Mendel’s dihybrid cross supported the hypothesis that each pair of alleles segregates independently of the other pairs during gamete formation known as LAW OF INDEPENDENT ASSORTMENT. Inheritance of one character has no effect on the inheritance of another. EXAMPLE: Independent assortment is seen in two traits in Labrador retrievers. Coat color AND eyesight (see next slide).

Mating of double heterozygotes (black coat, normal vision) Blind dog Blind dog Phenotypes Black coat, normal vision Black coat, blind Chocolate coat, normal vision Chocolate coat, blind Genotypes B_N_ B_nn bbN_ bbnn (a) Possible phenotypes of Labrador retrievers Mating of double heterozygotes (black coat, normal vision) BbNn BbNn Phenotypic ratio of offspring 9 black coat, normal vision 3 black coat, blind 3 chocolate coat, normal vision 1 chocolate coat, blind (b) A Labrador dihybrid cross Figure 9.9 Figure 9.9 Independent assortment of genes in Labrador retrievers.

Two trait cross (dihybrid cross) example Let’s consider two traits, hairline and finger length We’ll use: W for widow’s peak, and w for straight hairline S for short fingers, and s for long fingers A person who is WwSs (widow’s peak and short fingers) and a person who is also WwSs have children What are the gametes for each parent _____ , _____ , _____ and _____ Make a big Punnett square as illustrated on the following slide Figure the genotypes and phenotypes of the offspring

Dihbrid cross

Practice examples For each of the following genotypes, give all possible gametes WW WWSs Tt Ttgg AaBb For each of the following, state whether the genotype or a gamete is represented D Ll Pw LlGg (No two letters in a gamete can be the same letter of the alphabet.)

Using a Testcross to Determine an Unknown Genotype A testcross is a mating between An individual of dominant phenotype (but unknown genotype) A homozygous recessive individual © 2010 Pearson Education, Inc.

Two possible genotypes for the black dog: Testcross Genotypes B_ bb Two possible genotypes for the black dog: BB or Bb Gametes B B b b Bb b Bb bb Offspring All black 1 black : 1 chocolate Figure 9.10 Figure 9.10 A Labrador retriever testcross

Recessive Disorders Human genetic disorders are often recessive. Meaning you have to get ____ copies of the allele to have the disorder. Individuals who have the recessive allele but appear normal are CARRIERS of the disorder. Parents Offspring Dd Hearing Sperm Eggs D d (carrier) DD dd Deaf Figure 9.14

Genetic Testing Today many tests can detect the presence of disease-causing alleles. Most genetic testing is performed during pregnancy. Example: Amniocentesis Extracting fluid from around the fetus to check for genetic problems Genetic counseling helps patients understand the results and implications of genetic testing. Parents are given a choice as to what to do about the pregnancy or child.

VARIATIONS ON MENDEL’S LAWS Why do some of you have dark skin or light skin or some shade in between? Why do some plant flowers come in red, white or pink in color? Some patterns of genetic inheritance are not explained by Mendel’s laws. (Incomplete dominance and Codominance) Incomplete Dominance (in Plants and People) In INCOMPLETE DOMINANCE, F1 hybrids have an appearance in between the phenotypes of the two parents.

P Generation Red White RR rr Gametes R r F1 Generation Pink Rr Gametes Sperm R r R RR Rr Eggs r Rr rr Figure 9.18-3 Figure 9.18 Incomplete dominance in snapdragons. (Step 3)

Example of Incomplete Dominance in Humans Hypercholesterolemia (Review: what does hyper mean? __________) Is characterized by dangerously high levels of cholesterol in the blood. Is a human trait that is incompletely dominant. Heterozygotes have blood cholesterol levels about twice normal. Homozygotes for the trait have blood cholesterol levels about five times normal.

HH Hh hh Homozygous dominant for ability to make LDL receptors Heterozygous GENOTYPE Homozygous recessive for inability to make LDL receptors LDL LDL receptor PHENOTYPE Cell Normal Mild disease Severe disease HDL is “good” cholesterol, and LDL is “bad” cholesterol. If you are homozygous dominant (normal) your cells bind and remove LDL. If you are heterozygous or homozygous recessive, you are less able to remove bad (LDL) cholesterol. Figure 9.19 Incomplete dominance in human hypercholesterolemia.

ABO Blood Groups: An Example of Multiple Alleles and Codominance The A B O blood groups in humans are an example of codominance. More than 2 possible alleles exist in the population. People can have A or AB or O blood types. The AB blood type is the example of codominance. © 2010 Pearson Education, Inc.

Given the following genotypes, what would the phenotype (blood type) be? AA = blood type AB = blood type BB = blood type OO= blood type AO= blood type BO= blood type

If you are given the wrong blood, your immune system produces proteins called antibodies that can bind specifically to the wrong blood cells. The antibodies “know” your blood cells, and can detect the wrong blood cells. If the wrong blood is given, your antibodies will stick to the “invading” blood cells and cause the cells to clump together. This clumping would cause blood clots, which would lead to heart attacks and strokes. That is why you MUST have your blood type checked before being given blood!

Polygenic Inheritance Polygenic inheritance is the additive effects of two or more genes on a single phenotype. Most traits (and most diseases) are polygenic in humans!!! Example: Skin pigmentation in humans

Fraction of population Skin pigmentation Figure 9.22b Figure 9.22b A model for polygenic inheritance of skin color.

The Role of Environment Many human characters result from a combination of heredity and environment. Example: If you inherit the gene (or genes) that make you likely to develop lung cancer, how does your environment play a role in this? If you don’t smoke: If you do smoke: If you work in a factory or mine that produces toxic gasses: If you work at a computer in an office:

The dark color we see in these cats is a result of increased melanin production (the same thing that gives human skin color). Do dark colors absorb more heat or less heat?

SEX CHROMOSOMES AND SEX-LINKED GENES Any gene located on a sex chromosome is called a SEX-LINKED GENE. MOST sex-linked genes are found on the X chromosome. Few genes are found on the Y chromosome. RED-GREEN COLOR BLINDNESS is a common human sex-linked disorder. It is a trait found on the X chromosome. Since the Y chromosome is NOT homologous, it does not carry any gene related to color vision

Y X Colorized SEM Figure 9.29 Figure 9.29 The chromosomal basis of sex determination.

XNXN XnY XNXn XNY XNXn XnY Xn XN Y Xn Y Y XN XN XNXn XNY XNXN XNY XN Sperm Sperm Sperm Xn XN Y Xn Y Y XN XN XNXn XNY XNXN XNY XN XNXn XNY Eggs Eggs Eggs XN Xn Xn XNXn XNY XNXn XnY XnXn XnY Carrier female  colorblind male Normal female  colorblind male Carrier female  normal male Key Unaffected individual Carrier Colorblind individual Figure 9.31 Figure 9.31 Inheritance of colorblindness, a sex-linked recessive trait.