Mendelian Inheritance Biology In Focus Chapter 11 AP Biology 2014 Ms. Eggers

Learning Objectives  Define Mendel's law of segregation.  Contrast dominant alleles with recessive alleles.  Define genotype and phenotype.  Explain how the law of probability applies to genetics.  Describe how a Punnett square can be used to predict the results of a genetic cross.  Describe how a pedigree may be used to determine the mode of inheritance of a genetic trait.  List some well-known genetic disorders in humans.  Explain the inheritance pattern of traits where more than two alleles for the trait exist.  Define incomplete dominance and incomplete penetrance.  Define what is meant by pleiotropy.  Describe the inheritance of polygenic traits.  Explain X-linked inheritance and give some examples of X-linked traits in humans.

Gregor Mendel ( )  Austrian monk  Biology & math geek? Yup!  Explored heredity : the passage of characters (a heritable feature that varies among individuals) from parents to offspring.  Looked at TONS of pea plants for a variety of different traits and came to conclusions about the transmission of characters: Mendel ’ s Principles of Heredity.

Mendel’s choice of pea plants as his experimental model was key  Pea plants reproduce quickly  The mating of plants could be precisely controlled  Peas demonstrate multiple traits that occur in two, distinct forms

Crossing pea plants

Dichotomous traits in peas

The Monohybrid Cross  Monohybrid : the two plants crossed were both “ true-breeding ” but contrasting for one single trait.  True-breeding : all of the offspring display only one form of the trait.  Mating two true-breeding varieties is called hybridization.  What are the names of the various generations?

What did he conclude from the results of his monohybrid crosses?  First – in the F 1 generation the white flower disappeared. But it reappeared in the F 2 generation! This gave Mendel the notion that hereditary factors remain discreet – and do not blend. He called the trait present in the F 1 the dominant character and the trait that was disguised during the F 1 the recessive trait.

Hereditary factor = gene  Genes come in variations  The variation of a gene is called an allele  Genes/alleles for the same trait are located in the same position on homologous chromosomes

What else did he conclude from the monohybrid cross?  Second – when he counted the number of purple flowers and white flowers in the F2 generation, he saw 705 purple and 224 white flowered plants. He consistently saw that the dominant and recessive traits appeared in a 3:1 ratio. Another way of saying this is that ¾ of the plants had purple flowers and ¼ of the plants had white flowers.

In total, he looked at over 21,000 hybrids pea plants for these experiments!

Mendel knew his math…  Mendel took this information and said to himself, “ Self. I know a little bit about probability. If each parent has 2 copies of the “ hereditary factor ” for that trait but they only give one copy to their offspring (so the offspring gets one from each parent), then it’s just the flip of a coin which copy the offspring gets from each parent… So…

Genetics? Or calculating the odds in Vegas?  If a parent has genotype Aa (is heterozygous ), she can give either A or a to her offspring and its just a 50:50 chance – or ½.  Same for the other parent…  The probability that the offspring will get any combination is the product of the two separate probabilities (½ x ½ = ¼).

Vocabulary  Heredity : the passing of characters from parents to offspring  Genetics : the study of heredity  True-breeding : all of the offspring of a self-cross in plants display only one form of the character  P generation, F1 generation, F2 generation  Alleles : different versions of a gene  Dominant : the expressed form of a characteristic  Recessive : the form of the trait not expressed when the dominant allele is present  Homozygous : having two of the same allele  Heterozygous : having two different alleles  Genotype : refers to the combination of alleles  Phenotype : the physical appearance of a characteristic

This handy picture is a Punnett Square – and it shows what probability predicts.

Monohybrid cross ratios  The ratio of phenotypes in the F2 generation is 3:1 (or ¾ are the dominant phenotype and ¼ is the recessive phenotype).  The ratio of genotypes is 1:2:1 for AA:Aa:aa

Remember meiosis?

Mendel’s law of segregation  The two alleles for a character segregate (separate) when gametes are formed.  In other words, during meiosis I the homologous chromosomes separate so that each gamete only gets one copy of any chromosome/gene.

Dihybrid cross: two distinct traits at a time

Dihybrid cross ratios  Ratio of phenotypes is 9:3:3:1 of plants with both dominant traits: one dominant and the other recessive: one dominant and the other recessive the other way: and both recessive.  Again, probability shows why!

Think of each trait separately  For round vs. wrinkled seeds… in the F2 generation ¾ offspring will be round and ¼ will be wrinkled.  For yellow vs. green seeds… in the F2 generation ¾ offspring will be round and ¼ will be wrinkled.  So the chances of getting both round and yellow (both dominant) is ¾ x ¾ = 9/16. The chances of getting round and green is ¾ x ¼ = 3/16. The chances of getting wrinkled and yellow is ¼ x ¾ = 3/16. And the chances of getting both green and wrinkled (both recessive) is ¼ x ¼ = 1/16.  This gives a ratio of 9:3:3:1.

Each trait is separate – Mendel’s law of independent assortment.

What if you have a purple pea plant?  Can you know if it’s AA or Aa ? Not by looking…  But, you can do a test cross, which is crossing the unknown genotype with a pea plant that shows the recessive phenotype (and thus known genotype of aa ).

Dihybrid test cross

Lucky  Mendel got lucky. He chose the perfect experimental model with which to work. Peas show numerous traits with discreet completely dominant and recessive phenotypes.  Inheritance patterns for most traits are usually more complex

Incomplete dominance  Snap dragons exhibit incomplete dominance of flower color  Incomplete dominance : when offspring have a phenotype that lies somewhere between the two phenotypes of the parents  The Pp offspring make less red pigment than the PP plants & thus appear pink

Codominance  The two alleles affect the phenotype in separate, distinguishable ways

Homework  Explain how the inheritance of the human disease Tay-sachs demonstrates distinct dominant- recessive phenotype, incomplete dominance AND codominance at various levels. (p. 216)

Multiple alleles – human blood types

Pleiotropy  When a gene has multiple phenotypic effects

Even more complicated – when multiple genes interact to confer a given phenotype  Epistasis : the phenotypic expression of a gene at one locus alters than of a gene at a second locus  Lab coat color – the gene for pigment deposition ( E/e ) is epistatic to the gene that codes for black or brown pigment ( B/b )

Polygenic inheritance  Quantitative characters vary in a population along a continuum – such as human height or human skin color  This usually indicates polygenic inheritance = multiple genes contributing to a single phenotype

Pedigree analysis  Following human inheritance through generations can be informative  And some human traits do show simple Mendelian inheritance

Inherited human disorders  Cystic fibrosis : lethal recessive disorder. 1:25 people of European descent are carriers for this recessive allele. Results in defective chloride transport channels  Achondroplasia : a dominant trait  Huntington’s disease : a dominantly inherited disorder that results in neurodegeneration and death

Sickle cell anemia