basic principles of heredity

Basic Principles of HeredityPacket #18

Mendel

Vocabulary Word Introduction•Heredity

▫Transmission of genetic information from parent to offspring

•Genetics▫The science of heredity

Studies both genetic similarities and genetic variation

Vocabulary II• Genes

– Located on the chromosome– Composed of DNA

• Locus– The location of a gene on the chromosome

• Allele– Different form, of a particular gene, that is

located at a specific locus on a specific chromosome• Allele is used when investigation two or more

forms of a particular gene

Allele

Mendel’s Laws• When Mendel carried out his research,

the processes of mitosis and meiosis had not yet been discovered.

• Principle of Segregation– During meiosis, the alleles for each locus,

separate from each other– When haploid gametes are formed, each

contain only one allele for each locus– Segregation of alleles is a direct result of

homologous chromosomes separating during meiosis

Mendel’s Laws• Principle of Independent Assortment

– The random distribution of alleles, of different loci, into gametes

– Results in recombination• The presence of new gene combinations not

present in the parental (P) generation.– Independent assortment occurs because there

are two ways in which two pairs of homologous chromosomes can be arranged at metaphase I of meiosis.• The orientation of homologous chromosomes on the

metaphase plate determines the way chromosomes are distributed into haploid cells.

Mendel’s Laws

Mendel’s LawLaw of Independent Assortment

Vocabulary III•Dominant Allele

▫May mask the expression of the other allele known as the recessive allele There must be two alleles present

•Recessive Allele▫May only be expressed when paired with

another recessive allele

Homozygous vs. Hetereozygous•Homozygous Dominant

▫Two identical alleles that are in a dominant state

•Homozygous Recessive▫Two identical alleles that are in a recessive

state•Hetereozygous

▫Two different alleles One dominant One recessive

Genotype vs. Phenotype•Genotype

▫Composition of a specific region of DNA, in an individuals genome, that varies within a population

▫The allele composition found within a cell Allows the expression of the phenotype

•Phenotype▫The physical effect of a particular

genotype.

Genotype vs. Phenotype

Punnett Square•Punnett Square

▫A diagram used in the study of inheritance

▫Shows the result of random fertilization in genetic crosses.

Solving Genetics ProblemsTest/Monohybrid/Dihybrid Cross• Monohybrid Cross

– A cross, between parents (P generation), involving ONE allele

• Test Cross– A cross between individuals of an unknown genotype and a

homozygous recessive individual• Still involving ONE allele

• Dihybrid Cross– A cross, between parents (P generation), involving TWO

alleles.• The first generation of offspring

– F1 generation• First filial

• The second generation of offspring– F2 generation

• Second filial

Punnett Square• Example #1

▫ Sex determination• Sex is determined by sex

chromosomes▫ X & Y

• The Y chromosome determines male sex in most species of mammals▫ The Y chromosome

contains the SRY gene Sex reversal on Y gene

Punnett Square•Example #2

▫Monohybrid cross


▫ Test Cross


▫ Dihybrid cross

Blood Groups

Multiple Alleles•Three, or more alleles, can potentially

occupy a particular locus.▫A diploid individual any two of the three

alleles▫A haploid individual, or gamete, has only

one

Blood Groups II

Rh Factor• Determines whether someone has positive

or negative blood• A protein antigen that is on the surface of

blood cells and if that antigen is present, the individual is positive– A+; B+; O+; AB+

• If the antigen is not present, then the individual is negative– A-; B-; O-; AB-

Saturday, April 22, 2023

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Rh Factor II•If an RH-negative mother is exposed to

blood from an Rh-positive fetus, the mother’s blood will produce antibodies that will attack the blood of the fetus--potentially killing the unborn child.

•This is why, blood types should be determined before having children

•If, the male and female are negative, and positive, the mother must receive medication to prevent her immune system from attacking the child.


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Punnett Square•Example #5

▫Blood Type Cross We WILL NOT be doing Punnett Squares

involving the Rhesus factor.

Incomplete Dominance•Occurs when hybrids have an appearance

between the phenotypes of the parental varieties.▫The hetereozygote is intermediate in

phenotype▫Example

The color between red and white Pink

Incomplete Dominance

Punnett Square•Example

▫Incomplete Dominance

Codominance•Situation in which the phenotypes of both

alleles are exhibited in a heterozygote▫Hetereozygote simultaneously expresses

the phenotypes of both parents.•Example

▫Red Flower crossed with a White Flower The child will display flowers with red and

white spots Both alleles are exhibited

Punnett Square•Example #

▫Codominance

Epistasis•Epistatis occurs when one gene alters the

expression of another gene▫The genes are independent of each other


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Epistasis

Linkage• Each chromosome behaves genetically as if it

consisted of genes arranged in a linear order• Linkage is the tendency for a group of genes,

on the same chromosome, to be inherited together via crossing over

• Therefore, groups of genes on the same chromosome are linked genes.– Independent assortment does not apply if two

loci are linked close together on the same pair of homologous chromosomes.• Normally, they are passed on together.

– However, recombination of linked genes can result from crossing-over during Prophase I of Meiosis I

Linked vs. Unlinked•Recombination of unlinked genes =

Independent Assortment of chromosomes•Recombination of Linked genes =

Crossing Over

Linkage II• Measuring the frequency

of recombination between linked genes may provide an opportunity to construct a linkage map of a chromosome.

Distinguishing Between Independent Assortment and Linkage(Linked Genes)•Perform a two-point test cross

▫One individual must be hetereozygous for the linked genes

▫One individual must be homozygous recessive for the both characteristics

•Linkage is recognized when there is an excess of parental type offspring (majority) and a deficiency of recombinant type offspring are produced in the two-point cross.

Two Point Cross•Parent #1

▫BbVv Grey with normal wings

•Parent #2▫bbvv

Black with vestigial wings

Linked Genes

Two-Point CrossBV bv Bv bV

bv BbVv bbvv Bbvv bbVv

Expected Results

575 575 575 575

Actual Results

965 944 206 185

• Calculations– Parental Genotypes

• 965 (42%) +944 (41%) = 1909

• 1909/2300 = 83%– Recombinant Genotypes

• 206 (9%)+185 (8%) = 391• 391/2300 = 17%

– If independent assortment was to occur, the percentages would be 25% a piece.

– The recombinants arose because of crossing over

Gene Mapping• By measuring the

frequency of recombination between linked genes, it is possible to construct a linkage map of a chromosome▫ This is how scientists

were able to develop a detailed genetic map of Neurospora (fungus), fruit fly, the mouse, yeast and many plants that are particularly important as crops

Sex-Linked Genetics• Sex is determined by sex chromosomes

– X and Y• XX = female• XY = male

• The X chromosome contains many important genes that are unrelated to sex determination– These genes are required for both males and

females• A male receives ALL of his X-linked genes from

his mother while a female receives her X-linked genes from both parents.

Sex-Linked Genetics

Female Mammals• Display Dosage Compensation

– In females, only one of the two chromosomes is expressed in each cell

– Equalizes the expression of x-linked genes for both genders.• The other allele is inactive• Seen as a dark-staining Barr body at the edge of the

interphase nucleus.– A random event that occurs in each somatic cells

• A female that is hetereozygous expresses one of the alleles in about half her cells and the other allele in the other half

Dosage Compensation II•Mice and cats have several alleles that

code for coat color on the x-chromosome.▫Females that are hetereozygous for such

genes may show patches of one color in the middle areas of the other color. Variegation

Not always visible in other circumstances May require special techniques

Dosage Compensation

X-Linked Recessive Disorder• Males will show this trait if they have the

recessive allele on the X chromosome– Considered as hemizygous for the trait

• Females will show this trait if they have the recessive allele on both X chromosomes– Homozygous recessive

• Hemophilia– Inability to have clotting of blood– xh

• Color blindness– xc


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X-Linked Dominant Disorder•Baldness

▫XBXb

This female will not go bald due to lack of testosterone

▫XBXB

This individual will start to lose her hair in the future


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Pleiotrophy•The ability of one gene to have several

effects on different characteristics.▫Normally, can be traced to a single cause

Defective enzyme

Disorders caused by some form of alteration (mutation) on an autosome

Autosomal DisordersHuntington Disease• Caused by a rare autosomal dominant allele that

affects the nervous system▫ Gene found at one end of chromosome #4

• No symptoms appear until 30’s and 40’s• Symptoms

▫ Uncontrollable muscle spasms Degeneration of the nervous system

▫ Personality changes• Ultimately fatal 10-20 years after onset of

symptoms• No effective treatment has been found• Problem with symptoms appearing in the 30’s and

40’s▫ These individuals have children of their own before the

disease develops


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Autosomal DisordersSickle Cell Anemia• Caused by a change in polypeptides found in

hemoglobin– Hemoglobin is the protein that carries oxygen in red

blood cells– The recessive allele causes the change in the

polypeptide chain• Individuals that are hetereozygous display co-

dominance– Both alleles are expressed– Individuals are partially resistant to malaria

• Caused by Plasmodium, a protist (protozoan), carried by the Anepheles mosquito

• Mild Symptoms– Fatigue (feeling tired)– Paleness– Jaundice (Yellowing of the skin and eyes)– Shortness of breath


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Sickle Cell Anemia


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Autosomal DisordersPhenylketonuria (PKU)•Autosomal recessive disorder•Lack enzyme that converts amino acid

phenylalanine to another amino acid▫Tyrosine

•The excess phenyalanine is converted to toxic phenylketones ▫Damages the developing nervous system

•Can be screened for early in life and lifestyle changes made to prevent severe symptoms that result in mental retardation

Autosomal DisordersCystic Fibrosis•Autosomal recessive disorder•Gene responsible for the disorder

codes for a protein that transports chloride ions across cell membranes

•Defective protein, found in the epithelial cells lining the passageways of lungs, intestines, pancreas, liver, sweat glands ad reproductive organs result in the production of a thick mucus

•Leads to tissue damage•What are some treatments available?

Autosomal DisordersTay-Sachs Disease•Autosomal recessive disorder•Caused by abnormal lipid metabolism

in the brain•Results in blindness and severe mental

retardation•Symptoms begin in the first year and

normally result in death before the age of 5 years.

•Lack of enzyme results in the inability to break down a lipid in the brain

•Lipids build in the lysosomes•Lysosomes swell and burst causing the

nerve cells to malfunction

Ploidy•Degree of repetition of the basic number

of chromosomes•Diploidy

▫Chromosomes repeat 2X Remember, in humans, you have one copy of

a chromosome from the maternal father and one from the maternal mother


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Euploidy•“True” ploidy

▫Having 2 copies of each chromosome

Polyploidy•Definition

▫The presence of multiple sets of chromosomes

•Common in plants but rare in animals•Normally lethal in humans

Aneuploidy•Either missing, or having, extra copies of

certain chromosomes.•Trisomy

▫Indicates the individual has an extra chromosome

•Monosomy▫Indicates that one member of a pair of

chromosomes is missing


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Non-Disjunction• Causes trisomy or

monosomy• Causes

▫ Homologous pairs fail to separate During Anaphase I of

Meiosis I▫ Sister chromatids fail to

separate During Anaphase II of

Meiosis II


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Sex Chromosome AneuploidyTurner Syndrome•2n - 1

▫ 45 XO karyotype 44 autosomes + 1 X chromosome

There is the absence of a sex chromosome▫ No Barr bodies

• Female in appearance but their female sex organs do not develop at puberty and they are sterile▫ Ovaries degenerate in late embryonic life

• Short in stature• Shows normal intelligence but some cognitive

functions are defective• There are no Barr bodies

▫ Due to the lack of the other X chromosome


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Sex Chromosome AneuploidyKlinefelter Syndrome•2n + 1

▫47 XXY karyotype 44 autosomes + 3 sex chromosomes

There is an extra X chromosome▫One Barr body per cell

•Male in appearance and they too are sterile▫Male with slowly degenerating testes

•Female type pubic hair pattern•May have breast development


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Turner Syndrome vs. Klinefelter Syndrome


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Klienfelter Syndrome

Sex Chromosome AneuploidyXYY karyotype•Males that are usually fertile•Some are unusually tall with heavy acne•Others may have some mental disabilities•Predisposition to be more violent in

behavior•Gametes never YY or XY--meiosis is normal•After age of 35, extra Y chromosome often

degenerates and is not passed onto offspring


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Sex Chromosome AneuploidyXXX karyotype•Fertile females•May be some mental disabilities

▫Rare•Eggs will produce only X after meiosis--

not XX


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Autosomal AneuploidyDown SyndromeTrisomy 21•Caused by an extra copy of chromosome

#21▫There are three copies of chromosome #21 in

their somatic cells•0.15 percent of all live births•Growth failure and mental retardation•Big toes widely spaced•Congenital heart disease•Mean life expectancy is about 17 years and

only 8 % survive past age 40

Trisomy 21

Autosomal AneuploidyPatau SyndroneTrisomy 13•Multiple defects•Death is typical by the age of 3

Autosomal AneuploidyEdward’s SyndroneTrisomy 18•Ear deformities•Heart defects•Spasticity and other damage•Death is typical by the age of 1

▫Some may survive longer

Abnormalities in Chromosome StructureDisorders•The changes in the shape of the

chromosome may be due to either of the following▫Translocation▫Deletions▫Fragile sites

Translocation• A chromosome

fragment breaking off and attaching to a non-homologous chromosome▫ Reciporcal

translocation Two non-homologous

pairs exchange genetic information

• Can result in deletion and/or duplication of genes

Translocation Down Syndrome•4% of Down Syndrome cases•Individuals actually have 46 chromosomes•One of copies of chromosome #14 has

combined with chromosome #21▫The large arm of chromosome #21 has been

translocated to the large arm of another chromosome--usually chromosome #14

Deletion•The loss of part of a chromosome•The abnormal chromosome is known as a

deletion•Sometimes chromosomes break and fail to

rejoin

Cri du Chat Syndrome•Part of the short arm of chromosome #5

is deleted▫Breakage point varies from case to case

•Infants normally have a small head with altered features▫Moon face

•Infants have a distinctive cry that sounds like a cat mewing

•Infants normally survive childhood•Exhibit severe mental retardation

Fragile Sites•Weak points at specific locations in

chromatids•Appears to be a place where part of a

chromatid appears to be attached to the rest of the chromosome by a thin thread of DNA▫Have been identified on the X chromosome and

certain autosomes

Fragile X Syndrome•Fragile site occurs near the tip of the X

chromosome▫Where nucleotide triplet CGG is repeated many

more times than normal•Most common cause of mental retardation

Genetic Screening & Genetic Counseling•Genetic Screening

▫Identifies individuals who might carry a serious genetic disease Screening of newborns is the first step in preventative

medicine•Genetic Counseling

▫Provide couples, concerned about the risk of abnormality in their children, medical and genetic information

Screening

Pedigrees•Definition

▫A family tree that shows the transmission of genetic traits within a family over several generations.

•Pedigree Analysis▫Useful in detecting autosomal dominant

mutations, autosomal recessive mutations, X linked recessive mutations and defects due to genomic imprinting Genomic Imprinting

Expressions of a gene based on its parental origin

Pedigree Analysis

Homework•Bioinformatics•Proteomics•Aminocentesis•Chronic villus sampling (CVS)•Preimplantation genetic diagnosis (PGD)•Know how to discuss (argue for/against)

▫Genetic discrimination▫The Human Genome Project

basic principles of heredity

Documents

locussegregation of

particular genotype

particular locus

parents p generation

parental p generation

specific locus

y genepunnett squareexample

genetic crosses