Sample combobox Lightning

samplecombobox.app

<aura:application extends="force:slds">
    <aura:attribute name="contactFields" type="Contact" default="{sobjectType:'Contact',Gender__c:'Male',Partner_Gender__c:'Female'}" />
    <aura:attribute name="genderOptions" type="List" default="[{label:'Male',value:'Male'},{label:'Female',value:'Female'},{label:'Not Specified/Other',value:'Not Specified/Other'}]" />
    <lightning:combobox name="applicantgender" label="Applicant Gender" value="{!v.contactFields.Gender__c}" placeholder="Select Gender" options="{! v.genderOptions }" />
    <lightning:combobox name="partnergender" label="Partner Gender" value="{!v.contactFields.Partner_Gender__c}" placeholder="Select Gender" options="{! v.genderOptions }" />
    Applicant Gender: {!v.contactFields.Gender__c}<br />
    Partner Gender: {!v.contactFields.Partner_Gender__c}
</aura:application>