Unit 3: Genetics

Question 1

Autosomal dominant diseases

Answer 1

• Huntington’s disease #4
• Neurofibromatosis #17

Question 2

Autosomal recessive diseases

Answer 2

• Sickle cell anemia #11
• Cystic fibrosis #7
• Tay Saccs #15
• Phenylketonuria (PKU) #12

Question 3

Autosome Chromosome Number Defects

Answer 3

• Down’s syndrom #21
• Edward’s syndrom #18
• Patau syndrom #13

Question 4

Sex chromosome number defects

Answer 4

• Turner’s syndrome (X)
• Klinefelter’s syndrome (XXY)
• Jacob’s syndrome (XYY)
• Metafemale (XXX)

Question 5

X-linked recessive genetic diseases

Answer 5

• Hemophilia
• Fragile x-syndrome
• Colorblindness
• Muscular dystophy (Duchenne)

Question 6

2 types of chromosomes

Answer 6

• Autosome chromosomes
  
  • Chromosome pairs #1 to #22
  • No difference in the autosome chromosomes between the male & female
• Sex chromosomes
  
  • Chromosome pair #23
  • These are different between male & female
  • Genes on these chromosomes function in both sexes

Question 7

2 types of number defects

Answer 7

• Polyploid
• Aneuploid

Question 8

2 ways to study inheritance

Answer 8

• Inheritance of genes
  
  • Examines the inheritance of individual genes from parents to offspring
• Inheritance of chromosomes
  
  • Examines the inheritance of entire chromosomes from parents to offspring

Question 9

Alleles

Answer 9

• Versions of a gene
  
  • Dominant allele
    
    • Always expressed (seen)
    • Represented by a capital letter
  • Recessive allele
    
    • Hidden or masked by the dominant allele. The only time a recessive allele is expressed (seen) is when the individual has two recessive alleles
    • Represented by a lower case letter
• Notes
  
  • The properties and function of a protein depend on its amino acid sequence
  • The amino acid sequence is determined by the nucleotide sequence of the gene
  • Changing the sequence changes the properties of the protein

Question 10

Aneuploid

Answer 10

• Deviate from the normal diploid number by a factor of one
• Two types
  
  • Trisomy: having one extra chromosomes
  • Monosomy: having one less chromosome
• In humans, most aneuploid embryos are spontaneously aborted or result in a miscarriage.
• There are a few aneuploid conditions where the embryo can survive
  
  • Down’s Syndrome
  • Edward’s Syndrome
  • Patau Syndrome
• These disorders are caused by non-disjunction
  
  • Non-disjunction can happen either in oogenesis or spermatogenesis, but happens more often in oogenesis and increases with frequency as a woman ages

Question 11

Autosome number defect

• Description
• Causes

Answer 11

• Description
  
  • A condition caused by an abnormal number of autosome chromosomes
• Causes
  
  • Non-disjunction
    
    • The failure of chromosomes to separate properly in Anaphase I or Anaphase II
    • Results in gametes that do not have the correct haploid number
  • Polyspermy
    
    • Fertilization by more than one sperm

Question 12

Colorblindness

Answer 12

• Type: x-linked recessive
• Location: X chromosome
• Protein function
  
  • Functions in the eye to allow for visualization of the color red and green
• Phenotype
  
  • Cannot visualize the color red or green
  • Both colors appear as gray or white
• Frequency:
  
  • 8-12 % of Caucasian males are colorblind
  • About 1% of Caucasian females are colorblind
  • Data does not exist for other races or ethnic groups, but it is extremely rare in other groups

Question 13

Cystic Fibrosis

• Type
• Gene location
• Protein function
• Phenotype
• Frequency

Answer 13

• Type: Autosomal recessive
• Gene location: #7
• Protein function
  
  • To form a channel-like structure in the cell membrane to transport chloride outside of the cell
• Phenotype
  
  • Mucus builds up in the lungs and digestive organs
  • Causes difficulty breathing and inadequate digestion of food
  • Life span is about 35 years
• Populations with a high frequency
  
  • Caucasian: 1 in 25 are carriers
  • African American: 1 in 60

Question 14

Dihybrid

Answer 14

• An individual that carries two different ALLELES of one gene and two different alleles of another gene
• Dihybrid cross phenotypic ratio = 9:3:3:1
  
  • 9/16 of the offspring have the two dominant traits
  • 3/16 have one of the dominant traits with one of the recessive traits
  • 3/16 have the other dominant trait with the other recessive trait
  • 1/16 have both recessive traits
• Example
  
  • Character A (controlled by alleles A 1 and A2) and character B (alleles B1 and B2)
  • The dihybrid would have the genotype A1A2 B1B2, i.e. a double HETEROZYGOTE

Question 15

Dominant alleles

Answer 15

Mask the expression of recessive alleles

Question 16

Down’s syndrome

Answer 16

• Type: Autosome chromosome number defect
• Genotype: Trisomy #21
• Diploid Number : 47
• Occurrence: 1 in 700 births (risk increases with maternal age)
• Caused by: Non-disjunction in egg or sperm but is usually caused by the egg
• Phenotype:
  
  • Characteristic facial features (wide nose, wide set eyes, large forehead, flat face, small head, and large creases over the eyelids), mental retardation (is the most common cause of genetic mental retardation), heart defects, high risk of Alzheimer’s if survive past age 40.
  • Average life span: 35 years

Question 17

Edward’s syndrome

Answer 17

• Type: Autosome chromosome number defect
• Genotype: Trisomy #18
• Diploid Number : 47
• Occurrence: 1 in 5000 births (risk increases with maternal age)
• Caused by: Non-disjunction in egg or sperm but is usually caused by the egg
• Phenotype:
  
  • Severe physical and functional defects in the fetus.
  • Every system is the body is affected.
  • The majority of fetuses with Edward’s syndrome die before birth as a miscarriage or a stillbirth.
  • Of those that survive, most live only a few months.

Question 18

Environmental influences

Answer 18

• Humans can manipulate the environment to lessen the damaging effects of genetic defects
• Factors can range from chemicals in air or water pollution, mold, pesticides, diet choices, or grooming products.
• Examples
  
  • In humans, nutrition helps determine height

Question 19

Exceptions to Mendel’s Laws

Answer 19

• Incomplete dominance (curly hair)
• Codominance
• Multiple allels (ABO blood types)

Question 20

Family hypercholesterolemia

Answer 20

• Increased blood levels of low-density lipoprotein (LDL) cholesterol (“bad cholesterol”)
  
  • Incomplete dominance
  • Affects about 1 in 250 people
  • Increases the likelihood of having coronary heart disease at a younger age
  • Liver cells lack cholesterol receptors

Question 21

Female chromosome pair #23

Answer 21

• Determines genetic gender
• Female genotype: XX (Females have two X-chromosomes)
• Female karyotype is shown below (arrow shows two X chromosomes at pair #23)

Question 22

Fragile X-syndrome inheritance pattern

Answer 22

• Type: X-linked recessive
• Location: X chromosome
• Protein function
  
  • Most likely involved in the nervous system
• Phenotype
  
  • Developmental abnormalities (mental retardation & hyperactivity)
  • 2nd most common form of genetic mental retardation
  • Characteristic facial features that become more obvious as the person ages
    
    • Long and narrow face, large ears, prominent jaw and forehead, unusually flexible fingers, and enlarged testicles
• Frequency
  
  • 1 in 4,000 males born each year

Question 23

Gene

Answer 23

• DNA that determines traits by causing the production of a specific protein on a chromosome

Question 24

Genetic disease

Answer 24

• Mutation in one gene
• Inherited and have 2 alleles
  
  • The mutated allele
  • The normal, non-mutated allele
• Notes
  
  • Don’t confuse a Genetic disease with diseases that have strong genetic components such as cancer, heart disease, high blood pressure, & diabetes. These are not genetic diseases. They are diseases with a strong genetic influence.

Question 25

Genotype

Answer 25

• Alleles that are responsible for a trait
• Expressed in 2 letters since an individual has two copies of every gene
• 3 genotypes
  
  • Homozygous dominant: 2 dominant alleles. Example: HH
  • Homozygous recessive: 2 recessive alleles. Example: hh
  • Heterozygous: 1 dominant and one recessive allele. Example: Hh

Question 26

Gregor Mendel

Answer 26

• Austian monk (1822 - 1884)
• “Father of genetics”
  
  • 1st to study how traits are passed from parents to offspring
  • Through breeding experiments, able to make certain conclusions about inheritance

Question 27

Hemophilia

Answer 27

• Type: X-linked recessive
• Location: X chromosome
• Protein function: Blood clotting
• Phenotype
  
  • Blot cannot clot
  • Affected individuals can bleed to death from the smallest injury
  • A blood transfusion is necessary in order to supply the missing protein needed for blood clotting
• Frequency
  
  • 1 in 5,000 males born each year in US

Question 28

Homologous chromosomes

Answer 28

• A pair of identical chromosomes (same size, shape, and genes)
  
  • An individual has 2 copies of every gene (one copy came from the oocyte and the other copy came from the sperm cell)
  • Even though homologous chromosomes contain the same genes, the exact form of the gene may be different

Question 29

Huntington’s Disease

• Type
• Gene location
• Protein function
• Phenotype
• Frequency

Answer 29

• Type: Autosomal dominant
• Gene location
  
  • Chromosome #4
• Protein function
  
  • Somehow necessary for brain cell function
  • When mutated, the protein forms clumps inside nerve cells which prevent proper nerve cell function
• Phenotype
  
  • Balance & coordination problems and fatal dementia
  • Symptoms do not appear until after age 40
  • Many people are not aware they have the mutated gene and are not aware they have passed on this fatal dominant mutation to their children
• Frequency
  
  • ~ 10 in every 100,000
  • Some geographic regions have higher frequencies
    
    • Venezuela (700 per 100,000)
    • Tasmania, Wales & Sweden (due to founder effect)

Question 30

Inbreeding

Answer 30

• Mating of close relatives
• Changes gene frequencies of the population because close relatives are genetically similar
  
  • Increases the frequency of recessive alleles & individuals with the homozygous recessive genotype
  • Decreases the frequency of dominant alleles & individuals with the homozygous dominant genotype and the heterozygous genotype
• Offspring is more likely to share detrimental recessive alleles (ie for Cystic Fibrosis or Hemophilia)

Question 31

Incomplete dominance

Answer 31

• When neither allele is dominant
• Heterozygote is a “blend” of both alleles
• Example: Pink flowers
  
  • Key:
    
    • B = red flower
    • b= white flower
  • Genotypes:
    
    • BB = red flowers
    • bb = white flowers
    • Bb = pink (a blend of both red & white)
• Example in humans: AB blood type & wavy hair

Question 32

Jacob’s Syndrome

Answer 32

• Type: Sex chromosome number defect
• Genotype: XYY
• Description: male with two Y-chromosomes
• Diploid Number: 47
• Occurrence: not known
• Caused by: Non-disjunction in sperm
• Phenotype:
  
  • The majority of individuals have no phenotype problems.
  • May be taller than normal, have severe acne, and learning disabilities

Question 33

Karyotype

Answer 33

• A display of an organism’s chromosomes grouped by homologous pairs
  
  • Chromosomes are isolated from blood cells and the chromosomes are stained with dye
  • Chromosomes are spread on a microscope slide & photographed
  • Each chromosome is cut out of the print and homologous pairs are matched according to size, shape, and banding pattern
  • Chromosomes are lined up by number to produce the karyotype

Question 34

Klinfelter’s syndrome

Answer 34

• Type: Sex chromosome number defect
• Genotype: XXY
• Description: male with two X-chromosomes
• Diploid Number: 47
• Occurrence: 1 in 750 births (risk increases with maternal age)
• Caused by: Non-disjunction in egg or sperm but is usually caused by the egg
• Phenotype:
  
  • Lower than normal IQ
  • Sexually immature
  • Sterile due to lack of internal reproductive organs
  • Taller than normal
  • Large hands and feet
  • Develop feminine characteristics

Question 35

Law of Dominance

Answer 35

• One factor may mask the expression of the other factor

Question 36

Law of independent assortment

Answer 36

• Different factors for each trait separate independently

Question 37

Law of Segregation

Answer 37

• Factor pairs separate when producing sex cells

Question 38

Law of unit character

Answer 38

• “Factors” control inheritance & factors come in pairs

Question 39

Locus

Answer 39

The location of a gene on a chromosome

Question 40

Male chromosome pair #23

Answer 40

• Male genotype: XY
• Male karyotype is shown below (arrows point to X and Y and pair #23)

Question 41

Mendel’s 5 conclusions

Answer 41

1. Genes are responsible for traits
2. Genes exist in alternate forms (alleles)
3. An organism has 2 copies of a gene
4. Gametes have only one copy of each gene
5. One allele may hide another allele

Question 42

Mendelian traits

Answer 42

• A trait that follows Mendelian inheritance patterns
  
  1. The trait is controlled entirely by genes (no environmental influences)
  2. Each trait is controlled by only one gene (1 gene = 1 phenotype)
  3. Each gene has only two alleles, where one allele is dominant and the other is recessive
• Examples
  
  • Traits studied in pea plant experiments (ie seed color, flower color, etc.)
  • Widow’s peak, hitchhikers thumb, etc.

Question 43

Mendel’s 3 Laws of Hereditary

Answer 43

• Law of Unit Character
• Law of Dominance
• Law of Segregation

Question 44

Metafemale

Answer 44

• Type: Sex chromosome number defect
• Genotype: XXX
• Description: female with three X-chromosomes
• Diploid Number: 47
• Occurrence: not known
• Caused by: Non-disjunction in egg or sperm but is usually caused by the egg
• Phenotype
  
  • The majority of individuals have no phenotype problems
  • Some individuals may have fertility problems and repeated miscarriages due high risk of non-disjunction during formation of oocytes

Question 45

Monohybrid cross

• Description
• Generation definitions

Answer 45

• A cross between two parents possessing a pair of contrasting characters
  
  • P generation: the parents that begin the cross
  • F1 generation: offspring of the P generation
  • F2 generation: offspring produced between F1 generation (inbreeding)
• Example
  
  • In peas, a monohybrid cross would be crossing together a plant with purple flowers with a plant with white flowers. These two plants have all other traits the same (such as height & pea color)–the only difference between them is flower color

Question 46

Multifactorial trait

Answer 46

• A trait caused by genes and environmental influences
• Examples
  
  • Coloring in Siamese cats/Himalayan rabbits
    
    • Black fur is produced on body areas that are colder such as the ears, paws, & tip of the nose.
    • In areas where body is warmer, such as the torso or trunk, the fur is lighter (white in rabbits, cream-colored in Siamese cats)
  • Examples in humans
    
    • Height & weight
    • Depression
    • Substance abuse
    • Intelligence

Question 47

Multiple allelic traits

Answer 47

• A trait where the gene has more than two alleles
• Each person still has 2 copies of the gene, but there are several different versions of the gene possible
• Example: Blood type
  
  • Gene has three alleles: A, B, and O
  • There are 6 possible genotypes and 4 possible phenotypes
  • Genotype
    
    • AA Type A blood
    • AO Type A blood
    • BB Type B blood
    • BO Type B blood
    • AB Type AB blood
    • OO Type O blood
  • The A and B alleles are dominant over O
  • The A and B alleles are incompletely dominant to each other (incomplete dominance is discussed later in these notes)

Question 48

Muscular dystrophy (Duchenne)

Answer 48

• Type: X-linked recessive
• A group of similar X-linked recessive genetic disorders
  
  • Duchenne is the most common type
• Gene location
  
  • X chromosome
• Protein function
  
  • Is involved in maintaining muscle structure
• Phenotype
  
  • Beginning in early childhood, muscles begin to weaken usually starting in the legs and lower body
  • Affected individuals are unable to walk during by the teenage years
  • All muscles in the body, including the heart, are eventually affected
  • Average lifespan is about 35 years
• Frequency
  
  • 1 in 3500 males born each year

Question 49

Mutation causes

Answer 49

• Induced: they are induced by substances called mutagens
• Spontaneous: they just happen. The majority of mutations are spontaneous
• Notes
  
  • If a mutation occurs in the primary spermatocyte or primary oocyte, the mutation will be present in the gamete cells & can thus be passed to the offspring

Question 50

Neurofibromatosis (NF)

• Type
• Gene location
• Protein function
• Phenotype
• Frequency

Answer 50

• Type: Autosomal dominant
• Gene location
  
  • Chromosome #17
• Protein function
  
  • Controls mitosis in nervous system cells, skin cells, and bone cells
• Phenotype
  
  • Non-cancerous tumors form on nerve cells and skin cells
  • Causes skeletal defects due to uncontrolled mitosis in bones, bowing on one leg (due to different leg lengths), and learning disabilities
• Frequency
  
  • 1 in 2500-3500 births

Question 51

Non-Mendelian Traits

Answer 51

• A trait that does not follow Mendelian inheritance patterns (complex/unpredictable)
• 4 specific types
  
  • Multiple allelic traits
  • Polygenic trait
  • Multi-factorial trait
  • Incomplete dominance

Question 52

Patau syndrome

Answer 52

• Type: Autosome chromosome number defect
• Genotype: Trisomy #13
• Diploid Number: 47
• Occurrence: 1 in 10,000 births (risk increases with maternal age)
• Caused by: Non-disjunction in egg or sperm but is usually caused by the egg
• Phenotype:
  
  • Severe physical and functional defects in the fetus.
  • Every system in the body is affected.
  • The majority of fetuses with Patau syndrome die before birth as a miscarriage or a stillbirth.
  • Of those that survive, most live only a few months.

Question 53

Pedigree

Answer 53

• A chart of a family’s history with regard to a particular genetic trait
• Often used to determine the inheritance of a genetic disease

Question 54

Phenotype

Answer 54

• An individual’s actual appearance
• Phenotype is expressed in words
• Examples: “blue eyes”, “curly hair”, and “ability to digest lactose”

Question 55

Phenylketonuria (PKU)

• Type
• Gene location
• Protein function
• Phenotype
• Frequency

Answer 55

• Type: Autosomal recessive
• Gene location: #12
• Protein function
  
  • An enzyme that chemically digests the amino acid phenylalanine
• Phenotype
  
  • Since phenylalanine cannot be digested, it builds up in the blood
  • Causes brain damage and mental retardation
• Populations with a high frequency
  
  • 1 in 26 Turkish population
  • 1 in 26 Irish population
• Notes
  
  • All newborns are screened for this using a sample of blood. If PKU is identified shortly after birth, mental retardation can be prevented if a strict, very low protein diet plan is followed

Question 56

Polygenic trait

Answer 56

• Traits controlled by two or more genes
• Each gene contributes to the phenotype, and each gene has its own alleles
• Allows us to see diversity in the phenotypes in the offspring from two parents and the population as a whole
• Examples
  
  • Hair color, eye color, skin color, & height

Question 57

Polyploid

Answer 57

• Having extra “sets” of chromosomes
• Polyploid embryos do not survive in humans. They are spontaneously aborted soon after fertilization
• Most common polyploid situation is triploidy which is having three extra sets
  
  • In humans, this is would be 69 chromosomes total
  • (3n) Polyploidy not fatal in plants & some animals
  • Many commercially valuable food items are polyploid. As examples, seedless fruits, cultivated fruits (which are larger than their wild counterparts) and commercially farmed shellfish

Question 58

Product rule of probability

Answer 58

• The chance of 2+ independent events occurring together is the product (multiplication) of their chance of occurring separately

Question 59

Pure breeding (aka true breeding)

Answer 59

• A homozygous individual; either homozygous dominant or homozygous recessive

Question 60

Recessive genetic disease examples

Answer 60

• Key
  
  • H - normal allele
  • h - mutated allele
• Possible genotypes:
  
  • HH (homozygous dominant) - does not have the disease
  • Hh (heterozygous) - does not have the disease
    
    • Called carriers because they are carrying the mutation & can pass it on to offspring
  • hh (homozygous recessive) - has the disease

Question 61

Sex-influenced traits

Answer 61

• A trait caused by a gene that acts dominant in one sex but recessive in the other
• Sex of the individual determines if the gene acts dominant or recessive
• The gene is on an autosome chromosome, not a sex chromosome
• Example
  
  • Pattern baldness gene (often called “male pattern” baldness). This gene has two alleles:
    
    • Hair loss
    • No hair loss
  • Sex determines how the allele acts:
    
    • In males, the hair loss allele is dominant
    • In females, the hair loss allele is recessive

Question 62

Sex-linked genes

Answer 62

• A gene located on a sex chromosome
• 2 types
  
  • X linked:
    
    • Genes on the X chromosome
    • X chromosome is one of the largest chromosomes, therefore it contains over 2,000 genes
    • Function both sexes
    • Examples: color vision, blood clotting, metabolism of certain amino acids
  • Y linked
    
    • Genes on the Y chromosome
    • Y chromosome is one of the smallest, therefore it would contain a small number of genes
    • Very few genes have been discovered on this chromosome; for some of these genes, it has not been proven they are actually on the Y chromosome
    • Most of this chromosome is non-functional DNA (meaning it does not code for a functional product)

Question 63

Sickle Cell Anemia

• Type
• Gene location
• Protein function
• Phenotype
• Frequency

Answer 63

• Type: Autosomal recessive
• Gene location: #11
• Protein function
  
  • Forms the hemoglobin protein which is found in red blood
  • Allows red blood cells to transport oxygen in the blood for cells to use
• Phenotype
  
  • Cells become sickle or crescent-shaped
  • Means less oxygen is carried in the blood
  • Causes people to get tired easily
  • The sickled red blood cells clog blood vessels causing severe pain, organ/tissue damage, and internal bleeding
• Populations with a high frequency
  
  • 1 in 14 African-Americans, Africa, Spanish speaking islands such as Cuba and Puerto Rico, and Mediterranean countries
  • 1 in 625 Caucasians are carriers

Question 64

Sum rule of probability

Answer 64

• The chance of an event that can occur in more than one way is the sum of the individual’s chances

Question 65

Tay Sacchs Disease

• Type
• Gene location
• Protein function
• Phenotype
• Frequency

Answer 65

• Type: Autosomal recessive
• Gene location
  
  • Chromosome #15
• Protein function
  
  • An enzyme located in the lysosome of brain cells
  • Chemically digests lipids
• Phenotype
  
  • Lipids build up in the brain
  • Influences brain function and causes mental retardation, blindness, deafness, and seizures
  • Life span is 4 years
• Populations with a high frequency
  
  • 1 in 27 Jewish are carriers
  • 1 in 2500 non-Jewish are carriers

Question 66

Testis Determining Factor (TDF) gene

(aka SRY)

Answer 66

• A gene that has been proven to be on the Y chromosome
• Causes testosterone to be produced in the embryo at about 6-8 weeks after fertilization.
• The presence of testosterone causes the embryo to become a male (by causing male genitalia and male internal reproductive organs to form)
• Absence of testosterone results in the embryo becoming a female by default

Question 67

Testcross

Answer 67

• When an organism that shows the dominant phenotype, but an unknown genotype, is crossed with a homozygous recessive individual
• Sometimes used to determine the genotype of an individual with a dominant phenotype

Question 68

Trait

• Definition
• Types

Answer 68

• An inherited characteristic
• 2 types
  
  • Physical traits: traits we can see when looking at an individual. Examples in humans are hair color and eye color
  • Physiological traits: traits we cannot see when looking at an individual. These are traits controlling how the body functions. Examples in humans are the ability to clot blood or digest lactose

Question 69

Turner’s syndrome

Answer 69

• Type: Sex chromosome number defect
• Genotype: X
• Description: female with only one X-chromosome
• Diploid Number: 45
• Occurrence: 1 in 2500 births (risk increases with maternal age)
• Caused by: Non-disjunction in egg or sperm but is usually caused by the egg
• Phenotype:
  
  • Lower than normal IQ
  • Short stature
  • Web like neck
  • Sexually immature (do not develop female secondary sexual characteristics that normally appear during puberty)
  • Sterile due to lack of internal reproductive organs

Question 70

Why does the occurrence of recessive genetic diseases vary by race, religion, or ethnicity?

Answer 70

• People in distinctive groups often share certain versions of their genes, which have been passed down from common ancestors
• If one of these shared genes contains a disease-causing mutation, a particular genetic disorder may be more frequently seen in the group
• Examples
  
  • Sickle cell disease, which is more common in people of African, African American, or Mediterranean heritage
  • Tay-Sachs disease, which is more likely to occur among people of Ashkenazi (eastern and central European) Jewish or French Canadian ancestry

Question 71

XY female

Answer 71

• An XY female is an individual who has the genotype of a male (XY) but the phenotype of a female
• Caused by a mutation in the TDF (SRY) gene such that testosterone is not produced
• XY female is sterile due to the absence of internal reproductive organs such as ovaries
• Even though the X and Y chromosomes are members of chromosome pair #23, they are not homologous to each other. They are not the same size and shape and they do not contain the same genes
• Because the X and Y are not homologous, the number of X-chromosome genes is different between sexes. Females have two copies of X-chromosome genes (because they have two X-chromosomes). Males have one copy of X-chromosome genes (because they have only one X-chromosome)