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Course  announcements  

•  Problem  set  #4  posted  •  Midterm  #2  is  next  Thursday  (2/20)  •  Dr.  Hochmuth  will  lecture  today  and  Thursday  

Chapter 7: Genes and Inheritance

Family resemblance: how traits are inherited

Learning  objecFves  

•  By  the  end  of  this  topic,  students  should  be  able  to:  – Describe  the  mechanism  by  which  single  gene  traits  are  passed  from  parent  to  child  

– Define  dominant,  recessive,  and  codominant  alleles  and  understand  the  phenotypes  resulFng  from  pairing  of  these  alleles  

– Analyze  pedigrees  for  sex-­‐linked  traits  and  autosomal  dominant  and  recessive  traits  

Family resemblance: Your mother and father contribute equally to your

genetic makeup.

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What  biological  process  creates  gametes?  

 InteracFve  Poll    -­‐    Get  out  your  smart  phones!    Review  from  last  class.  

Meiosis  

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How can a single bad gene make you smell like a rotten fish?

Some traits are controlled by a single gene.

q Heredity •  The passing of characteristics from parent to

offspring through their genes

Selective Breeding: Observing Heredity

Insert new fig 7-4

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Traits that are determined by the

instructions a person carries at one gene are called single-

gene traits.

Are there multiple-gene traits too?

q Yes

q Let’s first examine the mechanism by which single-gene traits pass from parent to child.

q We’ll then expand this model of heritability.      

Mendel learned about heredity by conducting experiments.

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True-Breeding

Insert new figure 7-8 part 1

Mendel’s  Peas  

•  hWp://www.youtube.com/watch?v=Mehz7tCxjSE  

 

q A dominant trait masks the effect of a recessive trait.

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Three Ideas Mendel Used for Explaining This Pattern of Inheritance

1.  Each parent puts into every sperm or egg it makes a single set of instructions for building the trait.

Three Ideas Mendel Used for Explaining This Pattern of Inheritance 2. Offspring thus find themselves with two

copies of the instructions for any trait (called alleles).

Three Ideas Mendel Used for Explaining This Pattern of Inheritance

3.  The actual trait produced by an individual depends on the two copies of the gene that they inherit from their parents.

•  Homozygous and heterozygous

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Phenotypes and Genotypes

q The outward appearance of an individual is called their phenotype.

q Underlying the phenotype is the genotype. •  This is an organism’s genetic composition.

Observing an individual’s phenotype is not sufficient

for determining its genotype.

Genotypes

q Homozygous dominant

q Heterozygous

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How do we analyze and predict the outcome of crosses?

q Assign symbols to represent the different variants of a gene.

q Generally, we use an uppercase letter for the dominant allele and lowercase for the recessive allele.

Probability and chance play central

roles in genetics.

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Probability has a central role in genetics for two reasons:

q The first is a consequence of segregation.

q The second reason is that fertilization, too, is a chance event.

Chance is important in genetics. Probabilities

q Any gamete produced by an individual heterozygous for a trait has a 50% probability of carrying the dominant allele and a 50% probability of carrying the recessive allele.

Probabilities

q If a male is heterozygous for albinism (Aa) and a female is homozygous for albinism (aa), what is the probability that their child will be homozygous for albinism (aa)?

q Poll time!

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A test-cross enables us to figure out which alleles an

individual carries.

q  You would like to produce white alligators via a mating program.

q  The problem is that you cannot be certain of the genotype of your alligators.

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q  They might be homozygous dominant, MM, or they might be heterozygous, Mm.

q  In either case, their phenotype is normal coloration. q How can you figure out which of these two

possibilities is the actual genotype?

Worksheet  

Using pedigrees to decipher and predict the inheritance

patterns of genes.

Pedigree: A type of family tree

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Analyzing Which Individuals Manifest the Trait and Which Do Not

Sex-Linked Traits

Why do breeders value “pedigreed” horses and dogs

so much?

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A Trait’s Mode of Inheritance Is Not Always Completely Obvious

q Incomplete dominance or…

q The influence of the environment The translation of genotypes

into phenotypes is not a black

box.

Incomplete dominance and codominance: The effects of both

alleles in a genotype can show up in the phenotype.

Incomplete dominance, in which the heterozygote appears to be intermediate between the two homozygotes

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Codominance, in which the heterozygote displays characteristics of both homozygotes

What’s your blood type?

Some genes may have more than two alleles.

Multiple Allelism

q A single gene has more than two alleles.

q Each individual still carries only two alleles.

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Inheritance of the ABO Blood Groups

q A, B, and O alleles

q The A and B alleles are both completely dominant to O.

q The A and B alleles are codominant to each other.

q  Individuals can be one of four different blood types: A, B, AB, or O.

Why are people with type O blood considered “universal donors”? Why are those with type AB considered “universal acceptors”?

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Multi-gene traits

How are continuously varying traits such as height influenced by genes?

Polygenic Trait

q A trait that is influenced by many different genes

Additive Effects

What happens when the effects of alleles from multiple genes all contribute to the

ultimate phenotype

Sometimes one gene influences multiple traits.

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What is the benefit of “almost” having sickle-cell disease?

The SRY Gene

q “Sex-determining Region on the Y-chromosome”

q Causes fetal gonads to develop as testes shortly after fertilization.

q Following the gonads’ secretion of testosterone, other developmental changes also occur.

Why are more men than women color-blind?    

Sex-linked traits differ in their patterns of expression in males

and females.

If a man is color-blind, did he inherit this condition from his

mother, his father, or both parents?

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Environmental effects: Identical twins are not

identical.

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Drinking diet soda can be deadly if you carry a single bad gene (2

copies).

What gene is it and why is it so deadly?

Could you create a temporarily spotted Siamese

cat with an ice pack?

Why?

Some genes are linked together.

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Most traits are passed on as independent features: Mendel’s law

of independent assortment.

Red hair and freckles

Genes on the same chromosome are sometimes

inherited together.

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Why do most redheads have pale skin?

Linked genes: Genes on the same chromosome, maybe even

right next to each other

Segregation: You’ve got two copies of each gene but put only one copy in each sperm or egg.