Biology~Chapter 12 Inheritance Patterns & Human Genetics.

Biology~Chapter 12

Inheritance Patterns &

Human Genetics

I. Chromosomes & Inheritance

A. Thomas Hunt Morgan- early 1900’s-experiments with fruit flies

-observed 4 pairs of chromosomes

-noticed that 3 pairs were the same in males & females but that 1 pair was different.

- called these “sex chromosomes”.

B. Autosomes & Sex Chromosomes

• Sex chromosomes- contain genes that determine the gender of an individual.

• Autosomes- the remaining pairs of chromosomes that do not directly determine sex.

• In mammals

- 2 X chromosomes= FEMALE (XX)

- 1 X + 1 Y is a MALE (XY)

C. Males determine the sex of the offspring

-each sperm has an equal chance of having an X or a Y

-however- the only option for eggs is to receive an X chromosome.1:1 male to female ratio

II. Linked & Sex Linked Genes

–are pairs of genes that tend to be inherited together.

A. Linked genes

• Genes which are close together on same chromosome.

• Linked genes do not exhibit Mendel’s law of independent assortment

B. Sex-Linked traits

• are traits that are coded for by alleles on a sex chromosome.

• Genes found on the X chromosome are X-linked genes

• Since the X chromosome is larger- there are more X-linked than Y- linked traits.

NOTE: Since males have only 1 X- a male who carries the recessive allele will show the X-linked trait.

III. MutationsA. Mutation -a change in the nucleotide-base

sequence of a gene or DNA 1. Germ cell- occurs in gametes

- do not affect the organism itself2. Somatic Cell-occur in organism’s body cells

- may affect the organism (ex-cancer) - NOT inherited

3. Lethal Mutations-cause death, often before birth4. Beneficial Mutations- result in phenotypes that are beneficial .

B. Types of Chromosome Mutations1. Deletion- loss of a piece of chromosome due to

breakage.

2. Inversion – a chromosome piece breaks off, flips around backwards & reattaches

3. Translocation – a piece of chromosome breaks off & attaches to a non-homologous chromosome

4. Non-Disjunction – a chromosomes fails to separate from its homologue during meiosis.

Note human disorders that follow on next few slides:

Examples of Chromosomal Mutations: Deletion & Inversions

http://staff.jccc.net/PDECELL/evolution/mutations/mutation.html

An example of a human disorder with a deletion error:

• Cri du chat syndrome- – missing part of chromosome 5- – cry of infants is similar to that of a meowing

kitten, due to problems with the larynx and nervous system.

– About 1/3 of children lose the cry by age 2.

Types of non disjunction:• Trisomy- an extra chromosome leads to an individual with an extra

chromosome in every cell of his/her body.

• Monosomy- is a deficiency in number of chromosomes and is defined as only one copy of a chromosome that is normally present in two copies. When fertilized, the outcome is 45 chromosomes in total. Monosomies are less likely to survive when compared to trisomies.

What causes non-disjunction?

• The cause of non-disjunction is unknown. Non-disjunction seems to be a chance event. Nothing that an individual does or doesn't do during their reproductive years can cause these chromosomal changes. We do know that non-disjunction occurs more frequently in the eggs of women as they get older.

Non- Disjunction Disorders with an extra chromosome

(these folks have 47 instead of the normal 46 chromosomes in people):

• Down syndrome- extra 21

• Edward’s Syndrome- extra 18

• Patau syndrome – extra 13

Examples Non disjunction:• Extra #21= Down’s

Syndrome

• Extra #18= (most don’t live beyond 1st 7 months) This girl is now 6.)

http://starbulletin.com/96/03/25/news/story2.html

Sex-chromosome abnormalities may also be caused by non-disjunction.

• Klinefelters Syndrome- XXY ( extra X)

• Turners Syndrome – only 1 X- missing a second X (XO)

• Super males XYY• Any combination (up to XXXXY) produces maleness.

Males with more than one X are usually underdeveloped and sterile.

• XXX and XO women are usually sterile

C. Types of Gene Mutation

1. Point Mutation- the substitution, addition or removal of a single nucleotide, occurs within a single gene or segment of DNA

2. Substitiution- one nucleotide replaces another

3. Frameshift Mutation- if some nucleotides are deleted- whole segment moves

4. Insertion Mutation – one or more nucleotides added- also causes a frameshift

• Which type of gene mutation do you think would cause the most serious errors?

– Point mutation

– Substitution

– Frameshift

Example: Example: substitution errorsubstitution error

• Just 1 Amino Acid is a Substitution ERROR in Sickle Cell Hemoglobin

• Causes Hb to be sickle shaped instead of round- can’t fit into red blood cells & changes their shape too.

Sickle Cell - Disease or Trait:Sickle Cell - Disease or Trait:

• Normally, a person inherits two genes (one from each parent) that produce normal hemoglobin (hemoglobin A).

• A person with sickle cell traitsickle cell trait inherits one normal beta-globin gene (hemoglobin A) and one defective gene (hemoglobin S)

(this can actually be a good thing – it provides some protection from malaria, which infects RBCs!)

• A person with sickle cell diseasesickle cell disease inherits 2 bad copies- only makes HgS

IV. Human GeneticsA. Inheritance of Traits

1. Pedigrees - be able to draw & read these!

2. Patterns- we learn about genetic disease by looking at patterns of inheritance over several generations.

B. Genetic Traits & Disorders1. Polygenic Inheritance2. Complex Characters3. Multiple Alleles4. Incomplete Dominance5. X-linked Traits6. Sex- Influenced Traits7. Single Allele Traits

Inheritance of Traits

• Pedigree Pedigree - a diagram that

shows how a trait is inherited over several generations

• - see diagrams page 241 textbook

Reading a pedigree• males are represented by squares • females by circles.

• An individual who exhibits the trait , – (for example, someone who suffers from Marfan syndrome),

is represented by a filled symbol.

• A horizontal line between two symbols is a mating

HE

SHE

DAD MOM

Reading a Pedigree

• The offspring:– are connected to each other by a horizontal

line above the symbols – and to the parents by vertical lines.

Example pedigree:

• Is the mom or dad in generation I affected by a trait?• How many offspring are shown in generation II?• How many daughters & sons in generation II?• How many have the trait?• How many offspring does daughter #1 have? How

many have the trait?

Dominant traitsDominant traits • are passed on to a son or daughter EVEN if

only one parent has it.

• Every affected individual has at least one affected parent

• Affected individuals who mate with unaffected individuals have a 50% chance of transmitting trait to each child

• Two affected

parents may have

unaffected children.

• Recessive pedigrees:Recessive pedigrees:

• Where did the trait seen in generation III come from?

• Traits can be passed on to children if both parents,

even if they may seem "normal”, are carriers of the recessive trait.

Recessive traitsRecessive traits

• are passed on to children from both parents, although the parents may seem perfectly "normal."

• Characteristics of recessive pedigrees are: – An individual who is affected may have parents who are

not affected;

– All the children of two affected individuals are affected;

– In pedigrees involving rare traits, the unaffected parents of an affected individual may be related to each other.

Recessive pedigree:

If individuals 2 + 3 in generation 3 have more children- will they be affected?Can Individual #8 in gen. 3 have any unaffected children?Can individual #9 in gen 3 have affected children?

Example pedigree of cystic fibrosis

www.cfscreening.com.au/.../CF/CFInherited.shtml

1. Polygenic Inheritance

- traits influenced by several genes

(most human characteristics)

example- skin color is additive effect of 6 genes

B. Genetic Traits & Disorders

2. Complex Characters

• traits influenced by both genes & environment

• Example- height


3. Multiple Alleles-

• genes with 3 or more alleles.

– Example- human blood type. IA, IB, i• 3 alleles- both IA & IB are dominate over i,

Codes for sugars that are on blood cell surface.

• four different blood types- A, B, AB, O


• What are the 2 genotypes for “type B?• Which blood type can donate to all the others?

(universal donor?)• Which blood type makes antibodies (will clot against)

both A & B blood types?

4. Incomplete Dominance

• there is an intermediate phenotype• Human example- wavy hair is the

intermediate between straight & curly hair

(remember chapter 9 plant example: - pink 4 o’clock flowers,

In between red & white flowers)


5. X-linked Traits• Traits that are on the X chromosome. Traits that are on the X chromosome.

• Since males only have one X chromosome- they are affected more than females

• Females have 2 chances to get a good copy of the gene but males only get 1 chance.


Examples of X-linked traits

• Hemophilia- blood clotting disorder- boys may bleed to death

• Duchene muscular dystrophy -affects cardiac and skeletal muscle, as well as some mental functions.

• Fragile-X syndrome- part of X chromosome has errors- most common cause of mental retardation in males

• Red-green color blindness- males cannot see these 2 colors.

6. Sex- Influenced Traits

• males & females show different phenotypes with same genotype-

• example- male pattern baldness


7. Single Allele Traits

• trait controlled by 1 allele- Dominant or Recessive

Dominant Examples: • Achondroplastic dwarfism- dwarf size

• Polydactly is the presence of a sixth digit.

• Huntington's disease (also called Woody Guthrie's disease)-progressive destruction of brain cells after age 30

• Marfans Syndrome- progressive connective tissue disorder. (some cases are new mutations with no family history)

Single allele recessive traits:Single allele recessive traits:• Albinism lack of pigmentation in skin, hair, and eyes, -

Homozygous recessive (aa) people make no pigments• PKU (Phenylketonuria) - lack enzyme to use the

amino acid phenylalanine, buildups breakdown products leads to mental retardation. 1 in 15,000 infants has- treat with DIET! (no phenylalanine)

• Tay-Sachs Disease- degeneration of the nervous system. Children rarely survive past five years of age.

• Maple Syrup Urine Disease (MSUD) –like PKU amino acids leucine, isoleucine, and valine

More single allele recessive examples:

• Sickel Cell Anemia- single amino acid substitution in hemoglobin causes lack of oxygen to body

• Cystic fibrosis- body creates sticky thick mucus – chloride ions- detected in SWEAT TEST. - was once considered a fatal childhood disease- modern treatments can increase lifespan to 30 years plus.

C. Detecting Genetic Disease

1. Testing• Amniocetesis- Dr removes some amniotic fluid,

14th-18th week of pregnancy, analyze fluid for proteins & look at chromosomes- some risk of pregnancy loss

• Chorionic Villi Sampling- clip cells from placental area- 8-14th week pregnancy

2. Treatments- vary with disease

• Huntington’s Disease• Down’s Syndrome• Cystic Fibrosis• Duchenne muscular

dystrophy• Sickle Cell Anemia• Trisomy 18 (Edwards)• Tay-Sachs disease• Color blindness• PKU (Phenylketonuria)

• Fragile X syndrome

ThalassemiaMarfan syndromeBreast CancerHemophiliaCri du ChatPolydactylyMaple Syrup urine diseaseTurner SyndromeKlinefelter's syndrome, (XXY syndrome)Super males (XYY)Patau syndromeAlbinism

•See me about other topics

List of Human genetic disorders- Pick 1 for your project.

Biology~Chapter 12 Inheritance Patterns & Human Genetics.

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