BEYOND MENDELOther patterns of inheritance

Mendel’s laws

• 2nd Principle of Segregation—homologous chromosomes separate into different gametes. Each allele has equal probability of ending up in any given gamete.

• 1st Law of Dominance—allele that is always expressed when the organism is a heterozygote is the dominant allele. The other type of allele is NOT expressed.

How does meiosis sort chromosomes?

• 3rd Principle of Independent Assortment Genes on different chromosomes sort into gametes independently.

• In metaphase II, pairs of chromosomes line up down the middle. One chromosome goes to each new daughter cell. Only one.

• Haploid cells receive one of each type of chromosome, 23 in all.

• Which daughter cell receives which of the parent’s two homologous chromosomes is a RANDOM event.

Dihybrid

• A dihybrid is the result of a cross between individuals with 2 different forms for 2 genes.

• Try crossing peas with different seed color and different seed shape.

• Check your understanding• Color Yellow (dominant) YY x green (recessive) yy• All offspring are Yy with color________________• Shape Round (dominant) RR x wrinkled (recessive) rr• All offspring are Rr with shape_______________

• Apply the independent assortment principle to 2 genes on 2 different chromosomes.

Complete a dihybrid

• Separate alleles—each combination is equally likely to occur.

• Try FOIL method.• (½ Y + ½ y) * ( ½ R + ½ r) =

¼YR + ¼ Yr + ¼ yR + ¼ yr

• Place one of the gamete combinations in front of one of the squares.

• Pass out the letters in the same way for a monohybrid.

YR

Yr yR yr

YR

Yr

yR

yr

YyRr

YyRr

       

       

       

       

 YYRR YYRr  YyRR  YyRr 

YYRr  YYrr  YyRr   Yyrr

YyRR  YyRr   yyRR yyRr 

 YyRr Yyrr   yyRr  yyrr

Be sure one letter for each gene is included!

Look at the seeds from the cross• How many are yellow and

round?• How many are yellow and

wrinkled?• How many are green and

round?• How many are wrinkled and

green?• Write the ratio____________

 YYRR YYRr  YyRR  YyRr 

YYRr  YYrr  YyRr   Yyrr

YyRR  YyRr   yyRR yyRr 

 YyRr Yyrr   yyRr  yyrr

9:3:3:1

Complete dominance• Mendel did his work with a plant that had only complete

dominance• The seven genes in peas had one dominant allele and

one recessive allele.• The dominant allele does the ‘talking’ and the recessive

allele is hidden in a heterozygote.

The next plant Mendel tried did not have complete dominance. His experiments did not support his hypothesis. It took 34 years before other scientists recognized his work and added to it.

Incomplete dominance• The homozygous individual shows the form of the allele. RR is red.

• The heterozygote will show a blending of the two versions. Rr is pink.

• The presence of two different alleles result in a form that is an intermediate to the parental forms.

Incomplete dominance example• In humans, the texture of the hair shows incomplete

dominance. • Homozygous for curly, CC, results in curly hair. • Homozygous for straight, SS, results in straight hair. • The heterozygous state, SC, results in the intermediate

form with all wavy hair.• C and S are both incompletely dominant, neither one are

recessive. X S C

S    

C    

What are the percentages of the phenotypes?

___%, ___%, ___% LabelCurly/Wavy/Straight

Codominance• A trait that shows codominance has heterozygotes that

show the effect of both alleles.• Each allele is expressed in the organism• Color in cattle

• red x white red & whitespotted offspring

Sickle Cell Anemia• Sickle cell anemia results from a change in the

hemoglobin. A single base in the DNA changes and one amino acid is changed. The molecule becomes long.

• With two normal hemoglobin alleles HbA HbA, a person has all normal red blood cells.

• Heterozygotes HbA HbS, have both normal red blood cells and some long sickle cells. Heterozygous condition helps people survive malaria.

• With two sickle cells HbS HbS, the person has sickle cell anemia, a painful condition with clots.

• Which pattern of inheritance does this gene show?

Codominance: both red blood cell types are present.

Check your understanding.

• A white homozygous cow is crossed with a homozygous red bull and the calf is roan with red and white patches of fur. What type of inheritance is roan?

• Two parents with poor night vision have three children, one with normal night vision, one who is night blind, and one with poor night vision. What pattern of inheritance is this?

codominance

Incomplete dominance

Multiple alleles• Blood type includes four main phenotypes: A, B, AB, and

Omore than two alleles.• Each blood type has specific antigens on the cell

membrane, and it is named for the antigens on the cell.• A and B are codominant: IA or IB

• O is recessive: I• Type A has two genotypes. Name them.• Type B has two genotypes. Name them.• Type O has one genotype. Name it.

AA and Ao

BB and Bo

oo

Determine the paternity of the child• The mother is type A, and the father is type B. Can a type

O child possibly be their offspring?

(Hint: start with the child who

is recessive.)

• Could an AB father have a type O child? Why?

X ___________ ____________

___________    

___________    

X ___________ ____________

___________    

___________    

Check your understanding• Alleles of a single gene can interact with other alleles of the same gene or with the environment.

Name the type of inheritance• When heterozygous offspring look like one parent but not the other -

• complete dominance.• When heterozygotes show a phenotype unlike that of either parent -

• incomplete dominance.• When heterozygotes show characteristics of both parents

• co-dominance.

Resources• Monohybrids, 2014, The Biology Project, http://

www.biology.arizona.edu/mendelian_genetics/problem_sets/monohybrid_cross/09t.html