CHAPTER 12Chromosomes and Human

Inheritance

HUMAN CHROMOSOMES Diploid = 46, Haploid = 23 Sex chromosomes different than others

Not homologous pairs Female = two X chromosomes (XX) Male = one X & one Y chromosome (XY) Autosomes—non-sex chromosomes,

same in both genders

KARYOTYPING Culture cells,

stimulate mitosis, stop division at metaphase

Hypotonic solution swells cells, separates them & chromosomes

Take picture, cut & paste

AUTOSOMAL INHERITANCE

Autosomal DominantAllele expressed in

heterozygotesAA or Aa show

dominant phenotype

AUTOSOMAL DOMINANT Achondroplasia

Embryonic cartilage in skeleton doesn’t develop properly

“Dwarf”, average 4’ tall

Huntington’s DiseaseNervous system deterioratesSymptoms often not seen until

after 30Die in 40s or 50s

AUTOSOMAL INHERITANCE Autosomal

RecessiveRecessive

phenotype only shown with homozygous recessive

Heterozygous is “carrier”

AUTOSOMAL RECESSIVE Albinism

Lack of normal amounts of melanin (pigment) in body

Cystic fibrosis Thick mucus in lungs

& digestive tract Breathing & digestion

difficult Most common lethal

genetic disorder among caucasians

SEX DETERMINATION Humans

XX = FemaleXY = Male

SEX DETERMINATION Fetal development

7 weeks, “uncommitted”Y chromosome present male sex organsY chromosome absent female sex organs

SEX DETERMINATION Different species have

different systems X-Y

XX = Female, XY = Male

X-OXX = Female, X = Male

Z-WZW = Female, ZZ =

Male Haplo-Diploid

2n = Female, n = Male

X-LINKED INHERITANCE Non-sexual gene “linked” to a sex

chromosome (X) NOT present on Y chromosome Recessive & dominant alleles XR, Xr, Y XRXR, XRXr, XRY = dominant phenotype XrXr, XrY = recessive phenotype

X-LINKED INHERITANCE

XXB XY

XXB XY

XB YX

X

XB = Bald, recessive trait X, Y = Non-bald, dominant trait

XBXB

XBY

XXB XY

XB YXB

X

XXB XBY

XX XY

X YXB

X

XBXB = bald womanXXB = carrier womanXX = normal womanXBY = bald manXY = normal man

X-LINKED INHERITANCE Baldness Red-Green Color

Blindness Hemophilia A

Lack of certain clotting factors

Blood cannot clot, bleed uncontrollably

CHROMOSOME STRUCTURE Changes in physical structure of

chromosome Cause genetic disorders or

abnormalities Rare

Duplication Deletion Inversion Translocation

DUPLICATION DNA sequences are

repeated 2 or more times

Unequal crossing-over Broken piece of

chromosome attaches to homologous chromosome

Huntington’s Disease Affects coordination &

movement Affects mental

abilities, personality

DELETION DNA sequence deleted Unequal crossing-over Chemical damage Most cause serious

disorders or death Cri du chat syndrome

Severe developmental & neurological problems

Unusual cry of infant (“cry of the cat”)

Unusual physical appearance

INVERSION DNA sequence reverses No loss of DNA or

chromosome part No problem for carrier if

non-crucial gene Some may not know until

children have problem Chromosome 9

No health problems for parent

May increase risk of miscarriage

TRANSLOCATION Broken part of one

chromosome attaches to another

Usually reciprocal (both exchange broken parts)

Often cause reduced fertility

Severe problems rare Can include several

cancers

CHROMOSOME NUMBER Aneuploidy—one more one less

chromosome than normal Usually fatal for humans

Most miscarriages Nondisjunction—one or more pairs do

not separate during meiosis Polyploidy—cells with 3 or more copies

of one or more chromosomes Trisomic—cells with 2n+1 of one

chromosome, 2n of all others

NONDISJUNCTION

DOWN SYNDROME Only trisomy that reaches adulthood Extra copy of chromosome 21 Mostly through nondisjunction at

meiosis Abonormal mental, heart, and skeletal

development

SEX CHROMOSOME ABNORMALITIES Turner syndrome

Nondisjunction of sex chromosomes Only one X, noted as “XO” 98% of embryos miscarry Adults very short, but well proportioned Sterile, limited sex hormones

Klinefelter syndrome 2/3 Nondisjunction of sex chromosomes XXY Mostly normal, some learning disabilities Lower testosterone, higher estrogen “Feminized” male characteristics

HUMAN GENETIC DEVELOPMENTS Phenotype treatments

Phenylketonuria (PKU)—lack of enzyme, can’t convert certain amino acid, brain function problems

Restrict intake, can lead normal life Genetic screening

Detect alleles that can cause disorders Prenatal diagnosis

Amniocentesis—collect fluid from around fetus

Cells in fluid from fetus, can analyze for disorders