By Lisa Marie Meffert, PhD

Rice University

Heredity:

Pedigrees-Working Out Inheritance

Patterns

Genology - Lee Family of Virginia and Marylandc1886 Apr. 26.

Prints and Photographs Division,Library of Congress (LC-USZ62-90145)

BioEd Online

How is gender determined? (text p 318)

Recall that in humans the diploid # of chromosomes is 46 (23 pairs)

There are 22 pairs of homologous chromosomes called autosomes

The 23rd pair of chromosomes are different in males and females

How is gender determined? (text p 318)

These two x’mes are called the sex chromosomes.

Indicated by the letters X and Y Females are homozygous XX Males are heterozygous XY

Gender determination (cont’d)

Which chromosomes will determine the gender? The male determines gender Why? What is the expected ratio of males to females? Complete a punnett square (XX x XY)

XX x XY

X Y

X

X

XX

XX XY

XY

Sex Linked Traits

Traits controlled by genes located on the sex chromosomes are called sex linked traits (most often the X chromosome)

Y can’t cover up the effects Males either have it or not

Sex Linked Traits

Females can have it, not have it or be carriers Carriers can pass the gene, but do not exhibit the

characteristics of the gene More about this when we talk about pedigrees

Sex Linked Traits

Nondisjunction (p 271) The events of meiosis usually proceed

accurately Sometimes homologous chromosomes fail to

separate properly Anaphase I – chromosome pairs separate (1

to each daughter cell)

Nondisjunction (p 271) Nondisjunction – both chromosomes of a

homologous pair move to the same pole. One gamete has an extra chromosome

(n+1)

The other is short one chromosome (n-1)

http://www.ccs.k12.in.us/chsteachers/Amayhew/Biology%20Notes/mutations%20notes_files/image006.jpg

Meiotic Nondisjunction at Meiosis I AnimationTokyo Medical University Genetics Animations

Levels of Genetic Disorders

What are Genetic Disorders? List of disorders with info

Trisomy

Zygote with one normal gamete and one gamete with extra x’me 47 x’mes – Down Syndrome AKA – Trisomy 21

Organism with an extra chromosome often survives

Monosomy

Organisms are one or more chromosomes short – usually don’t survive

Cause of most chromosomal miscarriages E.g. Turner syndrome

Tetraploid

Changes in Chromosome Size

Fragile –X Results from a faulty crossover event

that results in a longer X chromatid. A child receiving this chromosome can

be male or female but mostly boys because it is a recessive trait to a normal X. Their faces are longer, have trouble with gait, many have learning differences or

disabilities and autism-like mannerisms.

Cri du Chat

1/20 000 live births, mostly girls Deletion of chromosome 5

http://learn.genetics.utah.edu/content/disorders/whataregd/cdc/

William’s Syndrome

1/7500 births Deletion of genes on chromosome 7 Elfin, perfect pitch, trouble spacial relationships,

cognitive processing difficulties, aortic defects

http://learn.genetics.utah.edu/content/disorders/whataregd/williams/index.html

Syndromes

Trisomy 21 – Down syndrome Trisomy 13 – Patau’s syndrome XO – Turner’s syndrome XXX – Trisomy X (metafemales) XXY – Klinefelter’s syndrome XYY – Jacob’s syndrome OY – lethal

Turner syndrome – XO monosomy.

Dwarfism Webbed neck Valgus of elbow. Amenorrhea

Klinefelter’s Syndrome - Trisomy XXY

testicular atrophy increase in gonadotropins in urine.

Jacob’s syndrome

Jacob's syndrome is a rare chromosomal disorder that affects males.

It is caused by the presence of an extra Y chromosome.

Males normally have one X and one Y chromosome.

Jacob’s syndrome

However, individuals with Jacob's syndrome have one X and two Y chromosome.

Males with Jacob's syndrome, also called XYY males

Patau’s syndrome

Fig 12.2 - Pedigree Chart

Family history that shows how a trait is inherited over several generations.

Carriers: those heterozygous for a trait. Can determine if

autosomal (occurs equally both sexes) sex-linked (usually seen in males) heterozygous (dominant phenotype) homozygous (dominantdominant phenotype,

recessive recessive phenotype)

Pedigree Symbols (see worksheet 103)

Dominant Pedigree

affected individuals have at least one affected parent

the phenotype generally appears every generation

two unaffected parents only have unaffected offspring

Recessive Pedigree unaffected

parents can have affected offspring

affected progeny are both male and female

Factors to Consider in Pedigrees

Is the trait located on a sex chromosome or an autosome? Autosomal – not on a sex chromosome Sex Linkage – located on one of the sex

chromosomes Y-linked - only males carry the trait. X-linked (recessive) - sons inherit the disease from normal

parents

How is the trait expressed? Dominant - the trait is expressed in every generation. Recessive - expression of the trait may skip

generations.

Pedigree Diagrams: I

Basic Symbols

Pedigree Diagrams: II

Basic Symbols for offspring and the expression of a trait. The offspring are depicted below the parents. Filling the symbol with black indicates the

expression of the studied trait.

Marfan’s Syndrome: An Example

Expressed in both sexes. Thus, autosomal.

Expressed in every generation. Thus, dominant.

Marfan’s: Genotype the Normal Individuals

Assign codes for the alleles. Code “m” for the recessive normal allele. Code “M” for the dominant allele for Marfan’s

syndrome.

Normal individuals must be “mm.”

Marfan’s: Genotype the Affected Individuals

Affected individuals must have at least one “M.”

Marfan’s: Parent-Offspring Relationships Possibilities for #1 and #2: Heterozygote (Mm) or

homozygous for “M?” If “MM,” all offspring from a normal mate should be

affected. Therefore, both must be heterozygotes.

Marfan’s: Parental Genotypes Known

“M” must have come from the mother. The father can contribute only “m.” Thus, the remaining genotypes are

“Mm.”

Albinism: An Example Expressed in both sexes at approximately equal

frequency. Thus, autosomal.

Not expressed in every generation. Thus, recessive.

Albinism: Genotype the Affected Individuals Assign codes for the alleles.

Code “A” for the dominant normal allele. Code “a” for the recessive allele for albinism.

Affected individuals must be homozygous for “a.” First generation parents must be “Aa” because they have

normal phenotypes, but affected offspring.

Albinism: Genotype the Normal Individuals Normal individuals must have at least one “A.”

Albinism: Parent-Offspring Relationships

#1 must transmit “a” to each offspring. The “A” in the offspring must come from the father. Normal father could be either heterozygous or homozygous for

an “A.”

**

Albinism: Parental Genotypes are Known

Both parents are heterozygous. Normal offspring could have received an “A” from either

parent, or from both.

Albinism: One Parental Genotype is Known

Only the genotype of the offspring expressing albinism are known. Normal offspring must have received an “a” from their affected father.

Hairy Ears: An Example Only males are affected. All sons of an affected father have hairy ears. Thus, hairy ears is Y-linked.

Hairy Ears: Female Sex Determination All females are XX.

Hairy Ears: Male Sex Determination All males are XY.

Hairy Ears: Gene on the Y Chromosome Code “H” indicates the allele on the Y

chromosome for hairy ears.

Hairy Ears: Wild-Type Allele for Normal Ears Code “+” indicates the allele on the Y

chromosome for normal ears.

Hemophilia: An Example In this pedigree, only males are affected, and sons

do not share the phenotypes of their fathers. Thus, hemophilia is linked to a sex chromosome–the X.

Expression of hemophilia skips generations. Thus, it is recessive. Extensive bruising

of the left forearm and hand in a patient with hemophilia.

Hemophilia: Expression of the Female Sex Chromosomes

All females are XX.

Hemophilia: Expression of Male Sex Chromosomes All males are XY.

Hemophilia: Genotype the Affected Individuals

Assign codes for the alleles. Code “H” for the recessive hemophilia allele. Code “+” for the wild-type normal allele.

Affected individuals must have an “H” on an X chromosome.

Hemophilia: Father-Daughter Relationship All daughters of an affected father receive an

X chromosome with the “H” allele.

Hemophilia: Genotyping the Normal Individuals Normal individuals must have at least one X

chromosome with the wild-type allele, “+.”

Hemophilia: Homozygous or Heterozygous? Only males affected Not Y-linked Skips a generation: recessive X-linked

Fig 12.2 - Discussion

Draw a punnet square for each generation

Assignment12.1 worksheets p 89, 97, & 104