MENDELIAN GENETICS AND MEIOSIS

Chapters 10 and 12

MENDEL’S LAWS OF HEREDITY

Gregor Mendel 19th century Austrian monk

1st studies of heredity genetics Traits characteristics that are inherited

Studied pea plants

Contrasting traits were easily seen Short generation time Many offspring per generation Sexes on 1 flower control of pollination Mathematical analysis of data

PHENOTYPES AND GENOTYPES

Phenotype

Physical appearance Expressed in words tall, short

Genotype

Genetic makeup Expressed in terms of alleles Allele form of a gene for a trait Dominant allele always expressed (T

tall) Recessive allele if present, may not

be expressed (t short)

More Terminology

Homozygous 2 identical alleles (TT, tt) Heterozygous (hybrid) 2 different

alleles (Tt) Different genotypes can have the same

phenotype Tall TT or Tt

MONOHYBRID CROSSESA single trait

Mendel crossed a tall plant with a short plant

Original parents P generation Offspring F1 generation All F1 were tall

F1 were allowed to self-pollinate

F2 75% were tall, 25% short 3:1 ratio

Mendel’s conclusion

Each organism has 2 factors (alleles) that control each trait

Rule of Dominance

When an individual is hybrid for a pair of contrasting traits, only the dominant trait can be seen

TT x tt Tt **recessive trait is masked**

Crossing 2 hybrids always results in 3:1

Law of Segregation

Pairs of alleles for a trait are separated during the formation of gametes and are recombined during fertilization

Tt T (egg) + t (sperm) Tt (zygote) Explains appearance of recessive traits in

subsequent generations

Dihybrid crosses 2 traits

Height and seed color

Law of Independent Assortment

Genes for different traits are inherited independently of each other

Exception genes on the same chromosome

Punnett squares

Predict the ratio of all possible results for a certain genetic cross

Not what will happen, but what could happen

Exact ratios are not seen in nature due to chance

Monohybrid cross height (Tt x Tt)

Dihybrid cross height, color(TtYy x TtYy)

TY Ty tY ty

TY TTYY TTYy TtYY TtYy

Ty TTYy TTyy TtYy Ttyy

tY TtYY TtYy ttYY ttYy

ty TtYy Ttyy ttYy ttyy

MEIOSIS Type of cell division in which

daughter cells receive only half the # of chromosomes of the

parent cell

GENES, CHROMOSOMES, AND

NUMBERS

Diploid vs. monoploid

Chromosomes occur in pairs 1 allele is on each of the paired chromosomes

DiploidCell with 2 of each kind of chromosome (2n)

Body cells (somatic)

MonoploidCell with 1 of each kind of chromosome (n) also called haploid

Gametes sperm and egg

Homologous chromosomes paired chromosomes

Each of a pair has genes for the same traits

They may carry different alleles

Why meiosis?

Associated with sexual reproduction 2 parents

Allows offspring to have the same number of chromosomes as parents No doubling of chromosome number

PHASES Meiosis I and II

2 separate divisions

Interphase

Replication of the chromosomes Same as in mitosis

Prophase I

Each pair of homologous chromosomes comes together to form a tetrad

This is known as synapsis

Crossing-over may occur at this point

Exchange of genetic material between nonsister chromatids

Results in genetic variation or mutation

Completely random and unpredictable

Metaphase I

Tetrads line up at cell equator metaphase plate

Anaphase I

Homologous chromosomes separate and move to opposite poles disjunction

Critical step without disjunction, gametes would have abnormal numbers of chromosomes

Telophase I

Cytokinesis forms 2 daughter cells Each cell has only 1 chromosome from

each homologous pair Each chromosome is still doubled

another division is required

Meiosis II

Identical to mitosis

Results 4 monoploid daughter cells

Let’s Review Meiosis!

Meiosis Overview

Meiosis and genetic variation

Crossing-over results in genetic recombination gene shuffling

Almost endless number of different possible chromosomes You are not the exact blend of your parents Explains Mendel’s results

MISTAKES IN MEIOSIS

Nondisjunction

Failure of homologous chromosomes to separate

Both chromosomes move to the same pole

1 cell has an extra chromosome 1 cell is missing a chromosome

Trisomy

A gamete with and extra chromosome fuses with a normal gamete

Zygote has 1 extra chromosome 47 instead of 46 in humans

Trisomy 21 Down syndrome

Monosomy

A gamete with a missing chromosome fuses with a normal gamete

Zygote has 1 missing chromosome 45 instead of 46 in humans

Lethal most of the time Turner syndrome XO

Polyploidy

Total lack of chromosomal separation Lethal in animals Frequent in plants

Larger and healthier fruits and flowers Plant breeders induce polyploidy by using

chemicals that cause nondisjunction

MENDELIAN INHERITANCE OF HUMAN TRAITS

Chapter 12

Pedigree

Graphic representation of genetic inheritance

A chart showing familial relationships and patterns of trait inheritance

Sample pedigree chart

Squares maleCircles femaleFilled in afflictedBlank not afflictedHalf filled in carrier

RECESSIVE HEREDITY2 copies of allele are needed

(Most genetic disorders)

Cystic fibrosis (CF)

Formation and accumulation of mucus in lungs and pancreas

Due to a defective protein 1 in 25 white Americans carry the allele Resistance against tuberculosis

Tay-Sachs disease

Buildup of lipids in brain cells (lethal) Missing the gene coding for an enzyme Amish and eastern European Jews

Phenylketonuria (PKU)

Accumulation of phenylalanine (amino acid) in brain cells

Causes mental retardation Missing the enzyme needed to break

down the amino acid Standard test for all newborn infants PKU mothers can damage unborn child

Sickle-cell anemia

Abnormally shaped red blood cells Mutation in hemoglobin gene Most afflicted don’t survive childhood Carriers are more resistant to malaria Africans

Dominant heredity

1 allele is needed Tongue rolling Hitchhiker’s thumb Huntington’s chorea

Degeneration of brain cells lethal Onset of symptoms at 30-50 years of age 50% chance of passing on the allele

COMPLEX PATTERNS OF INHERITANCE

Incomplete dominance

Dominant allele is only partially expressed when recessive allele is present

Genetic blending

Four-o’clock flowersRed (RR) x White (RR)

Codominance

Both alleles are expressed equally Cattle red coat and white coat

codominant Hybrid roan (mixture of red and white

hairs)

Multiple alleles

More than 2 possible alleles within a species

Human blood type 3 possible alleles

Sex determination

Sex chromosomes only unmatched pair All other are autosomes 22 pairs

Female XX Male XY It is the male that determines the sex of

the offspring

Sex-linked traits

Controlled by genes on the X chromosome

Males are more likely to be afflicted Females may be carriers Color blindness Hemophilia Male pattern baldness *

Polygenic inheritance

Trait controlled by 2 or more genes Not expressed in 2 contrasting forms, but

varies between the 2 extremes Human height

Gene linkage

Genes located on the same chromosome Inherited together

Environmental influences

External temperature, light, nutrition, infectious agents

Internal hormones, age, sex

INHERITANCE OF HUMAN TRAITS

Human blood typescodominance and multiple alleles

3 possible alleles IA and IB are codominant i is recessive

6 possible genotypes IAIA or IAi type A IBIB or IBi type B IAIB type AB ii type O

Sex-linked traits

Red-green color blindness Most common

Hemophilia Commonly missing clotting factor VIII

Polygenic inheritance

Eye color, skin color, height Almost all human traits

Changes in chromosome number

Autosomes Down syndrome Sex chromosomes

Turner syndrome XO Klinefelter syndrome XXY OY lethal

DETECTING GENETIC DISORDERS

Karyotype

Comparing specimen chromosomes to normal chromosomes

Amniocentesis

Sample of free-floating fetal cells Make a karyotype

Genetic Screening

Compare your genes to known disease markers