BIOL 2311 Chapter 13 (Genes, Chromosomes, and Human Genetics)

autosomal inheritance

patterns of inheritance of genes not on a sex chromosome

gene

a region of DNA in a chromosome that codes for a particular protein or RNA

allele

a particular form of a gene

genotype

a listing of the alleles of particular genes in an individual

phenotype

the observable traits of an individual

homozygous

having two of the same allele

heterozygous

having two different alleles

dominant allele

an allele that produces its phenotype in heterozygous and homozygous genotypes

recessive allele

an allele that produces its phenotype only in homozygous genotypes

pure line

individuals of the same phenotype that, when crossed, always produce offspring with the same phenotype - pure line individuals are homozygous for the gene

hybrid

offspring from crosses between homozygous parents with different genotypes

reciprocal cross

phenotypes of the male and female are reversed compared with a prior cross

reciprocal cross purpose

to test if the sex of the parent influences transmission of the trait

testcross

cross of a homozygous recessive individual and an individual with the dominant phenotype but unknown genotype

testcross purpose

to determine whether a parent with a dominant phenotype is homozygous or heterozygous

X-linked gene

a gene located on the X chromosome

Y-linked gene

a gene located on the Y chromosome

linked genes

genes that are near each other on the same chromosome

locus

location of a gene on a chromosome

linkage

linked genes can be inherited together

backcross

cross of F1 generation with parental generation

backcross purpose

to identify an elite type of genotypes by checking on gamete formation

testcross vs. backcross

all test crosses are back crosses, but not al back crosses are test crosses

Thomas Hunt Morgan’s experiments

Fruit fly (Drosophila melanogaster), parent w/ red eyes and normal wings + parent w/ purple eyes and vestigial wings

Thomas Hunt Morgan’s Experiment Results

expected results: 25% of each offspring

actual results: 47% + 42% (parental phenotype), 5% + 5% (recombinants)

proved that eye color and wing shapes are linked genes

genetic recombination

process in which two homologous chromosomes exchange segments with each other by crossing-over

recombination frequency

percentage of test cross progeny that are recombinants, dependent on distance between linked genes - closer genes have greater chance of being linked (# of recombinants/# of total gametes)

RF = 0.50 for unlinked genes

linkage map

shows relative locations of genes using recombination frequencies

units for linkage map

map unit (mu) or centimorgan (cM) equals 1% recombination frequency

homogametic sex

XX female

heterogametic sex

XY males

SRY gene

a gene on the Y chromosome that determines human sex, active SRY gene causes testes development and degeneration of female structures

pedigree

family records of genotypes and phenotypes of past generations

hemophilia

an X-linked gene, causes blood clotting to not happen, Queen Victoria was a carrier

dosage compensation mechanism

early during embryonic development, inactivates one of two X chromosomes in most body cells of females, Barr body formation, same X is inactivated in all descendants of the cell

Barr body

a X chromosome is condensed into a tightly coiled state, inactivates the genes on that X chromosome

Chromosomal mutations

changes in chromosome structure or # of chromosomes

Chromosome structure changes causes

DNA breaks and the fragments are lost or attached to the same or different chromosomes

Deletion mutation

a segment is lost from a chromosome, can be debilitating if missing segment contains genes essential for normal development or cellular functions

deletion mutation example

heterozygous deletion of human chromosome 5 typically have severe intellectual disability, a variety of physical abnormalities, and a malformed larynx (cri-du-chat syndrome)

bees/wasps sex chromosomes

no sex chromosome, sex is dependent on being haploid or diploid

some insects sex chromosomes

XX females and XO males

birds/butterflies/reptiles sex chromosomes

ZZ males ZW females

duplication mutation

broken segment is inserted into its homologous chromosome, likely to be detrimental, can occur due to uneven crossing-over

duplication mutation and evolution

since genes are duplicate, one of the copies can mutate into new forms without affecting basic functions, duplication mutations have been important sources of evolutionary change

translocation mutation

the broken segment is attached to a non homologous chromosome, in many cases it is reciprocal, most common type of mutation, involved in many cancers

translocation mutation example

chronic myelogenous leukemia (CML), Philadelphia chromosome happens when regulatory ABL gene of chromosome 9 fuses with BCR gene of chromosome 22

inversion mutation

the broken segment reattaches to the same chromosome but reversed, similar effects as translocation

euploids

individuals with a normal set of chromosomes

aneuploids

individuals with extra or missing chromosomes, generally embryos are so abnormal that they are naturally aborted

Down syndrome

nondisjunction causes extra copy of chromosome 21 (trisomy 21), primarily in women, more likely as maternal age increases, characterized by short stature and moderate/severe mental disabilities

aneuploidy of sex chromosomes

usually tolerated unlike autosomal aneuploidy

turner syndrome

XO, no Barr bodies, females, underdeveloped ovaries and breasts, normal intelligence, normal genitalia, short

Klinefelter syndrome

XXY, 1 Barr body male, underdeveloped testes, normal intelligence, some development of breasts

XYY syndrome

XYY, no barr bodies, normal male, taller than average

Triple X syndrome

XXX, 2 barr bodies, normal female, normal or slightly diminished mental function

polyploids

individuals with more than normal number of chromosomes, often due to failure of spindle function during mitosis, also can be due to multiple eggs fertilizing one egg

polyploid plants are usually hardier and more successful than diploids

polyploidy is usually lethal for animals

triploids (3 copies of each chromosome), tetraploids, hexaploids, etc.

monoploids

only one of each chromosome, lethal in most animals, tolerated in plants, some wasps/ants/bees are monoploid due to being from unfertilized eggs

crisscross inheritance

in family line, trait passes from male → female → male or vice versa

autosomal recessive inheritance

both sexes, can skip generations, can be carriers, only homozygous recessive show symptoms

autosomal recessive examples

sickle-cell disease, cystic fibrosis, phenylketonuria (PKU), albinism

autosomal dominant inheritance

both sexes, trait does NOT skip generations, homozygous or heterozygous dominant are affected

autosomal dominant examples

achondroplasia, progeria, myotonic dystrophy, hypotrichosis

X-Linked Recessive Traits

more males are affected than females, males always transmit to daughters but never to sons, can skip generations, can be carrier, females need 2 copies of the recessive allele, males only need 1 copy

X-linked recessive trait examples

red-green color blindness, hemophilia, Duchenne muscular dystrophy (DMD)

X-linked Dominant traits

both sexes, does NOT skip generations, affected males always transmit to daughters and never to sons, affected females always transmit to progeny

X-linked dominant trait examples

constitutional thrombopathy, hereditary enamel hypoplasia, Rett syndrome

genetic counseling

predicts possibility that a child’s genes will be negatively affected

percentage of newborns with mutant alleles

1%-3%

percentage of child patients in hospitals that are due to inherited disorders

10%-25%

prenatal diagnosis

cells from developing embryo or surrounding tissue are tested, amniocentesis or chorionic villus sampling, can detect >100 genetic disorders

amniocentesis

before 12 weeks, cells are obtained from amniotic fluid

chorionic villus sampling

between 10-12 weeks, cells are obtained from embryonic portions of the placenta

genetic screening

testing for inherited disorders after a child is born so that preventative measures can be taken, most US hospitals test newborns for PKU

non-mendelian patterns of inheritance (2 types)

cytoplasmic inheritance, genomic imprinting

cytoplasmic inheritance

inheritance patterns of DNA in chloroplasts or mitochondria, uniparental (usually maternal) inheritance, non-mendelian gene ratios

maternal inheritance

zygote receives most of its cytoplasm from female parent

Leber’s hereditary optic neuropathy (LHON)

mitochondrial gene mutation caused by mutation of any one of the genes for eight electron transfer system proteins, affects optic nerve, leads to progressive vision loss, most common inherited mitochondrial disorder, typically affects young males

genomic imprinting

a genes expression is dependent on whether the allele came from the mother or father

95 imprinted genes of humans and 120 of mice have been identified

silent allele/imprinted allele

the inherited allele that is not expressed due to genomic imprinting

insulin-like growth factor 2 (Igf2) gene

mice, only paternal allele has effect, maternal copy is imprinted

genomic imprinting mechanism

DNA in the area that controls gene expression is modified by adding methyl (-CH3) to cytosine nucleotides which prevents gene expression

happens during sperm production for paternally imprinted genes and during egg production for maternally imprinted genes

inherited imprints must be erased in the germ cell before new imprinting occurs