Chromosomes and Human Genetics-Marked Up(2)

8/2/2019 Chromosomes and Human Genetics-Marked Up(2)

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Chromosomes and Human

GeneticsChapter 20


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Genes and chromosomes- areview

Particular locus for each gene

A diploid cell has homologous pairs of

chromosomes; one from each parent

Crossing over occurs in meiosis

Genes in a diploid cell have two alleles

Independent assortment of chromosomes


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Some traits are inheritedtogether though linkage

Linkage between genes if the distance between them isshort Less likely to be separated during crossing over Tend to stay together/inherited together


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The sex chromosomes

Human sex chromosomes X chromosome

XX: femaleMost genes deal with nonsexualcharacteristics

Genes referred to as X-linked genes

Y chromosomeXY: male

SRY gene Genes referred to as Y-linked genes


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Which parent determines thesex of the baby?


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X-inactivation

X inactivation Most or all of one of a females X

chromosomes turned off soon after firstdivision of zygote

Condenses into a Barr body

Incontinentia pigmenti X-linked disorder Darker patches of skin color are

visible in tissue where the mutated Xchromosome is active


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Inheritance of genes on the sexchromosomes

X-linked disorders

Usually affects males. Why?


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Most X-linked traits arerecessive

Pattern of inheritance:


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Rest of the semester

Today through next Wed-continue with unit ongenetics

Thursday/Friday next week poster presentations

Monday Dec 12th

-in class video and worksheet Wednesday Dec 14th-quiz #7

Web-based documentary plus associatedworksheet will be assigned on Monday and due

at end of semester Health packet due at end of semester

(Wednesday December 21)


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Examples of X-linked disorders

Hemophilia Hemophilia A

Most common form Mutation in the gene for the clotting protein factor

VIII

Queen Victorias family had higher frequency ofhemophilia due to marriage between closerelatives

Duchene muscular dystrophy

Mutated form of muscle support proteindystrophin Muscles degenerate, lose ability to walk Usually die by age 20 from cardiac or

respiratory failure


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Inheritance of genes on theautosomes

Many genetic disorders are autosomalrecessive

Pattern of inheritance:


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Examples of autosomalrecessive disorders

Cystic fibrosis Faulty CFTR gene (inherited from

both parents) Fatal usually before the age of 30 Symptoms

Thickened mucus: clogs airways Infections

Treatments Physiotherapy; thumping on the backand chest to loosen mucus

Antibiotics to cure infections


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Examples of autosomal dominantdisorders

Huntington Disease Progressive degeneration of nervous system

Symptoms appear after age 30; fatal

Mutation on chromosome 4


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Sickle cell anemia and sicklecell trait

Sickle-cell anemia- homozygous recessive One amino acid substitution in hemoglobin

Val instead of glu

Pleiotropic effects Relatively common for a recessive geneticdisorder. Why?

Sickle cell trait-heterozygous Heterozygote advantage Confers resistance to malaria


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02.5%

Distribution ofmalaria caused byPlasmodium falciparum(a parasitic unicellular eukaryote)

Frequencies of thesickle-cell allele

2.55.0%

7.510.0%

5.07.5%

>12.5%

10.012.5%


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PedigreesFemaleMale

Individuals with the trait


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Chromosomal abnormalities

Mutation

Translocation

Changes in chromosome number


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Translocation


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Changes in Chromosome number

Aneuploidy Embryo does not have an exact multiple of

23 chromosomes

Polyploidy Embryo has three, four, or more sets of 23

chromosomes

Chromosome numbers can change duringmitosis, meiosis, or fertilization


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Changes in Chromosome number:nondisjunction

Nondisjunction One or more pairs of chromosomes fails to

separate during cell division Increases with a womans age If such a gamete is fertilized

Trisomy Monosomy

Down syndrome; trisomy 21 Mental retardation, abnormal skeletal

development, motor functions delayed, andweaker muscles


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Changes in Chromosome number:nondisjunction in sex chromosomes

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Chromosomes and Human Genetics-Marked Up(2)