Complex Patterns of Heredity

Chapter 12

Human Inheritance

• Humans have 23 pairs of chromosomes

• These are made of about 100,000 genes

• Scientists usually study disease causing genes because they can easily be traced

• They often prepare a Pedigree – a family record that shows how a trait is inherited over several generations.

Pedigree

• Carriers – usually, Heterozygous; they do not express the recessive allele, but they pass it along to their offspring.

Square = Male

Circle = Female

No shading = normal

Shaded = displays trait

Half/Half = Carrier

PedigreeA Pedigree of Hemophilia in the Royal Families of Europe

Simple Dominant Heredity

• Many traits are inherited just as the rule of dominance predicts.

• Remember that in Mendelian inheritance, a single dominant allele inherited from one parent is all that is needed for a person to show the dominant trait.

Patterns of Inheritance

• Phenotypes repeated in a predictable pattern from generation to generation

• Genetic Disorders – diseases or debilitating conditions that have a genetic basis

Patterns of Inheritance

• Traits controlled by a Single Allele– More than 200 human traits are determined by

a single dominant allele• Ex. Huntington’s Disease

– More than 250 other traits are determined by homozygous recessive alleles

• Both parents must have this allele in order for their offspring to have the disease.

• Ex. Cystic Fibrosis & Sickle cell anemia

Inheritance Patterns• Recessive Allele Inheritance Patterns

– Unaffected parents can have affected offspring– The phenotype can skip a generation– Individuals with no signs of the allele can be

carriers

• Dominant Allele Inheritance Patterns– Affected offspring must have at least one affected

parent– The phenotype appears in every generation without

skipping– Two unaffected parents have no affected offspring

Sometimes Heredity Follows Different Rules

• Incomplete Dominance: Appearance of a third phenotype

• Codominance: Expression of both alleles

• Multiple phenotypes from multiple alleles

• Sex determination

• Sex-linked inheritance

• Polygenic Inheritance

Incomplete Dominance• With incomplete dominance, a

cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits.  – RED Flower x WHITE Flower ---

> PINK Flower

• R = allele for red flowers W = allele for white flowers red x white ---> pink RR x WW ---> 100% RW

Codominance• The genetic gist to codominance is pretty

much the same as incomplete dominance.  A hybrid organism shows a third phenotype --- not the usual "dominant" one & not the "recessive" one ... but a third, different phenotype. 

• In COdominance, the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms.

Codominance

• Example

red x white ---> red & white spotted

With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together.

• R = allele for red flowers W = allele for white flowers red x white ---> red & white spotted RR x WW ---> 100% RW

Examples of Codominance• A very very very very very common

phenotype used in questions about codominance is roan fur in cattle.  Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together).  A good example of codominance. Another example of codominance is human blood type AB, in which two types of protein ("A" & "B") appear together on the surface of blood cells.

Codominance and Blood Types• Blood transfusion can only take place

between two people who have compatible types of blood.

• Human blood is separated into different classifications because of the varying proteins on the surface of blood cells.

• These proteins are there to identify whether or not the blood in the individual's body is it's own and not something the immunity system should destroy.

Patterns of Inheritance

• Traits controlled by Multiple Alleles– Controlled by 3 or more alleles of the same

gene that code for a single trait– Ex. Blood types

• A & B are codominant – both are expressed when together; and both are dominant to O.

• A person can only have type O blood if they receive the “O” allele from both parents.

Human Blood Types

ABO Blood type and genetics

• The protein's structure is controlled by three alleles;

• i, IA and IB. – i, the recessive of the three, – IA and IB are both codominant when paired together. – If the recessive allele i is paired with IB or IA, it's expression is hidden and is not shown. – When the IB and IA are together in a pair, both proteins A and B are present and

expressed.

• The individual's blood type is determined by which combination of alleles he/she has. – There are four possible blood types in order from most common to most rare: O, A, B

and AB. – O blood type represents an individual who is homozygous recessive (ii) and does not

have an allele for A or B.

• Blood types A and B are codominant alleles. – Codominant alleles are expressed even if only one is present. The recessive allele i for

blood type O is only expressed when two recessive alleles are present. – Blood type O is not apparent if the individual has an allele for A or B.– Individuals who have blood type A have a genotype of IAIA or IAi and those with blood

type B, IBIB or IBi,– An individual who is IAIB has blood type AB.

Blood type Chart

Blood type practiceUse a Punnett Square!

1. A woman has type A blood. Her father has type O blood. The woman marries a man with type O blood. What is the chance that they will have a child with type A blood?

2. What is the chance that the couple from question 1 will have a child with type AB blood?

*Show me your answers when you are finished. Keep these in your notes!

Multiple Phenotypes from Multiple Alleles

• Although each trait that we have studied so far only has two alleles, it is common for more that two alleles to control a trait in a population

• For instance, Pigeons – three colors possible (red, blue, chocolate)

• However, each pigeon can have only two of these alleles

• Complete P.S. Lab 12.2 to observe multiple alleles in how coat color in rabbits is inherited.

Sex determination

• Remember that in humans the diploid number of chromosomes is 46, or 23 pairs.

• There are 22 matching pairs of homologous chromosomes called autosomes.

• The 23rd pair differs in males and females, they determine the sex of an individual (sex chromosomes) – X females (XX)– Y males (XY)

Complete a punnett square to determine the expected ratio of males to females produced given their possible gamete contribution

Sex-linked inheritance

• Traits controlled by genes located on sex chromosomes are called sex-linked traits

• Read about Thomas Hunt Morgan’s research with fruit flies on pg 325. Complete a punnett square to show how the allele for red eye color is a sex-linked trait.

Patterns of Inheritance

• Polygenic Traits– Most human characteristics are controlled by

several genes (2 or more)– Ex. Skin color – 3 to 6 genes– Ex. Eye color– Some are also affected by the environment

• Ex. Height – nutrition and disease

Eye Color Activity

• http://www.athro.com/evo/gen/genefr2.html

Patterns of Inheritance

• Sex-Linked Traits– Are found only on the X chromosome– Ex. Colorblindness (recessive)– Ex. Hemophilia (recessive)

Patterns of Inheritance

• Sex-Influenced Traits

• Influenced by male or female sex hormones

• Ex. Patterned Baldness– Homozygous baldness-both will lose hair– Heterozygous-men will lose hair but women

will not

Nondisjunction

• Nondisjunction is the failure of homologous chromosome pairs to separate properly during meiosis. The result of this error is a cell with an abnormal (too few or too many) number of chromosomes.

Nondisjunction

Patterns of Inheritance

• Disorders due to Nondisjunction– Monosomy (45 Chromosomes)– Trisomy (47 Chromosomes)

• Trisomy-21 (Down’s Syndrome)

– Klinefelter’s (XXY)-male w/ some female traits– Turner’s (XO)-female appearance– Single Y chromosome do not survive

Environmental Effects

• Genes are inherited from parents, but sometimes their expression is modified by environmental factors.

• An example is the snowshoe hare we discussed earlier in the year-these hares have dark fur in the summer and white fur in the winter.

Snowshoe Hare

Detecting Human Genetic Disorders

• Genetic Screening – examination of genetic makeup– Karyotype: a picture of chromosomes

grouped in pairs and arranged in sequence.

– Screening of Blood: look for certain proteins• Genetic Counseling-medical guidance

informing of problems that could affect their offspring.

Human Karyotype

Detecting HumanGenetic Disorders

– Amniocentesis: removal of small amount of amnionic fluid surrounding the fetus

– Chorionic Villi Sampling: tissue that grows between the mother’s uterus and the placenta (between the 8th and 10th week)

– Screening Immediately after Birth:• Ex PKU (Phenylketonuria)-body cannot

metabolize the amino acid phenylalanine– Special diet lacking phenylalanine

Karyotyping

• http://www.biology.arizona.edu/human_bio/activities/karyotyping/karyotyping2.html